Discussion:
UTCDateAndTime discussion + Q&A
(too old to reply)
Chris Muller
2018-10-22 21:30:52 UTC
Permalink
Hi Dave,

Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.

A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
million-dollar question:

What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
There are two reasonable answers to this question:

1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).

We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
UTCDateAndTime? When I do:

DateAndTime now asDate = Date today

I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..

I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my
David T. Lewis
2018-10-23 01:02:57 UTC
Permalink
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.

In classic Chronology:
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00

In UTCDateAndTime Chronology:
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00

So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.

The likely fix is to change DateAndTime>>midnight:

DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)

But fixing that triggers one other test failure so I need to sort that
out before posting anything.

Tha
David T. Lewis
2018-10-23 03:21:58 UTC
Permalink
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort that
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime reposi
Chris Muller
2018-10-26 20:54:28 UTC
Permalink
Hi Dave, I took a look at Chronology-Core-dtl.30, it looks like a good change.

Now we just need something to obtain a canonical date from
DateAndTime's similar to Chronology's #beCanonical. BUT, given the
confusion we've had about #asDate in the past, what if we changed
#asDate to do the "expected" behavior -- answer the canonical version
--, and added, say, #asLocalDate to preserve the not-so-common
version?

- Chris
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort that
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository
David T. Lewis
2018-10-26 23:39:29 UTC
Permalink
Post by Chris Muller
Hi Dave, I took a look at Chronology-Core-dtl.30, it looks like a good change.
Now we just need something to obtain a canonical date from
DateAndTime's similar to Chronology's #beCanonical. BUT, given the
confusion we've had about #asDate in the past, what if we changed
#asDate to do the "expected" behavior -- answer the canonical version
--, and added, say, #asLocalDate to preserve the not-so-common
version?
Hi Chris,

That sounds reasonable enough to me, although I would defer to you and
others as to what is the preferred default because I really don't have
any applications that would be impacted one way or the other.

What's your feeling overall regarding UTCDateAndTime? Does it seem like
a worthwhile change from your point of view? And to my main worry, is
there anything about the different layout of instance variables that is
likely to cause problems for Magma, or for applications that use other
databases for persistence? I think that it will be OK, but it's better
to ask now than to be sorry later.

Thanks,
Dave
Post by Chris Muller
- Chris
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort that
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository in Chro
Chris Muller
2018-10-27 03:26:20 UTC
Permalink
Post by David T. Lewis
What's your feeling overall regarding UTCDateAndTime? Does it seem like
a worthwhile change from your point of view? And to my main worry, is
I would expect the storage and performance advantage of UTCDateAndTime
to be muted in 32-bit Squeak. Chronology breaking down the "point" of
a point-in-time into 3 SmallIntegers -- days, seconds, nanos (days
a.k.a., 'jdn') means there's always just three object pointers for
that. 50-bits for #utcMicroseconds in a 32-bit image, however, may be
nearly as expensive.

But since SmallIntegers run well beyond 50-bits in 64-bit Squeak, I
would expect a slight performance and storage advantage.

There's also the claimed understandability advantage, which is
something, but somewhat offset by the conversion / compatibility
issues users must face and endure. "Worthwhile" is definitely the key
question we need to decide. For me, it seems worth it in theory
because it seems like it shouldn't be too hard to convert. The only
way to find out is to attempt to convert one of my most date-intensive
applications (including database) see how it goes, see what issues
arise.
Post by David T. Lewis
there anything about the different layout of instance variables that is
likely to cause problems for Magma, or for applications that use other
databases for persistence? I think that it will be OK, but it's better
to ask now than to be sorry later.
There is the layout, there is also the hash calculation. Could you
make it the same as in Chronology? It could ease migration, but we
want it to remain fast.

We need to think about how different migrations could and should work.
Bulk conversions? Lazy migrations? We may need some way for apps to
be able to (quickly) ask if they're running the new UTCDateAndTime or
the old Chronology, to help support those...

- Chris


- Chris
Post by David T. Lewis
Thanks,
Dave
Post by Chris Muller
- Chris
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort that
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository in Chronology-Core-dtl.30.
Dave
David T. Lewis
2018-10-27 14:18:22 UTC
Permalink
Hi Chris,
Post by Chris Muller
Post by David T. Lewis
What's your feeling overall regarding UTCDateAndTime? Does it seem like
a worthwhile change from your point of view? And to my main worry, is
I would expect the storage and performance advantage of UTCDateAndTime
to be muted in 32-bit Squeak. Chronology breaking down the "point" of
a point-in-time into 3 SmallIntegers -- days, seconds, nanos (days
a.k.a., 'jdn') means there's always just three object pointers for
that. 50-bits for #utcMicroseconds in a 32-bit image, however, may be
nearly as expensive.
But since SmallIntegers run well beyond 50-bits in 64-bit Squeak, I
would expect a slight performance and storage advantage.
To check performance, you can use the DateAndTimePerformance package
in the http://www.squeaksource.com/UTCDateAndTime repository. I think
you will be pleased with the results, although I have only run it on
Linux so it would be good to check Windows and OSX also.
Post by Chris Muller
There's also the claimed understandability advantage, which is
something, but somewhat offset by the conversion / compatibility
issues users must face and endure. "Worthwhile" is definitely the key
question we need to decide. For me, it seems worth it in theory
because it seems like it shouldn't be too hard to convert. The only
way to find out is to attempt to convert one of my most date-intensive
applications (including database) see how it goes, see what issues
arise.
Post by David T. Lewis
there anything about the different layout of instance variables that is
likely to cause problems for Magma, or for applications that use other
databases for persistence? I think that it will be OK, but it's better
to ask now than to be sorry later.
There is the layout, there is also the hash calculation. Could you
make it the same as in Chronology? It could ease migration, but we
want it to remain fast.
The hash in the UTCDateAndTime version is:

DateAndTime>>hash
^utcMicroseconds hash

I don't know if this is a good hash function, although it at least
has the virtue of simplicity.

I expect we could make it compatible with the existing function if
that is a better thing to do, although it might have some performance
impact.
Post by Chris Muller
We need to think about how different migrations could and should work.
Bulk conversions? Lazy migrations? We may need some way for apps to
be able to (quickly) ask if they're running the new UTCDateAndTime or
the old Chronology, to help support those...
Some kind of test like "DateAndTime instVarNames size" should work.

Dave
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything appears
to be essentially the same as original Chronology except the on-going
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of Dates,
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local representation
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a duration, the
start time of the date should align with the timezone of the DateAndTime
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same timezone offset as the receiver."
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort that
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository in Chronology-Core-dtl.30.
Da
Eliot Miranda
2018-11-21 04:02:49 UTC
Permalink
Hi Chris,
Post by Chris Muller
Post by David T. Lewis
What's your feeling overall regarding UTCDateAndTime? Does it seem like
a worthwhile change from your point of view? And to my main worry, is
I would expect the storage and performance advantage of UTCDateAndTime
to be muted in 32-bit Squeak. Chronology breaking down the "point" of
a point-in-time into 3 SmallIntegers -- days, seconds, nanos (days
a.k.a., 'jdn') means there's always just three object pointers for
that. 50-bits for #utcMicroseconds in a 32-bit image, however, may be
nearly as expensive.
But since SmallIntegers run well beyond 50-bits in 64-bit Squeak, I
would expect a slight performance and storage advantage.
There's also the claimed understandability advantage, which is
something, but somewhat offset by the conversion / compatibility
issues users must face and endure. "Worthwhile" is definitely the key
question we need to decide. For me, it seems worth it in theory
because it seems like it shouldn't be too hard to convert. The only
way to find out is to attempt to convert one of my most date-intensive
applications (including database) see how it goes, see what issues
arise.
The simplicity of the large integer utcMicroseconds representation trumps
all the nonsense of breaking it down into sub components. In any case the
32-bit VM is communicating time up to the image as 64-bit large integer
microseconds anyway. Not decomposing gives much faster instantiation, and
very simple arithmetic (simply compare the utcMicroseconds).

Note that it would be trivial to extend the representation with the
decomposed elements, an d these could be nil initially and instantiated on
demand. Forcing the large integer arithmetic to decompose on every
instantiation would kill any performance advantage one might expect to get
from using immediate instead of there large integer. And of course in
64-bits, utcMicroseconds is an immediate anyway.
Post by Chris Muller
Post by David T. Lewis
there anything about the different layout of instance variables that is
likely to cause problems for Magma, or for applications that use other
databases for persistence? I think that it will be OK, but it's better
to ask now than to be sorry later.
There is the layout, there is also the hash calculation. Could you
make it the same as in Chronology? It could ease migration, but we
want it to remain fast.
We need to think about how different migrations could and should work.
Bulk conversions? Lazy migrations? We may need some way for apps to
be able to (quickly) ask if they're running the new UTCDateAndTime or
the old Chronology, to help support those...
- Chris
- Chris
Post by David T. Lewis
Thanks,
Dave
Post by Chris Muller
- Chris
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that
"just
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
works" -- not having to go research how to properly install it
left me
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything
appears
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
to be essentially the same as original Chronology except the
on-going
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am
to
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am
to
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of
Dates,
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local
representation
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
of 22-Oct-2018 that everyone else recognizes in my local area?
which
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a
duration, the
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
start time of the date should align with the timezone of the
DateAndTime
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same
timezone offset as the receiver."
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort
that
Post by David T. Lewis
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository in Chronology-Core-dtl.30.
Dave
--
_,,,^..^,,,_
best, Eliot
Chris Muller
2018-11-22 05:00:41 UTC
Permalink
HI Eliot,
Post by Chris Muller
There's also the claimed understandability advantage, which is
something, but somewhat offset by the conversion / compatibility
issues users must face and endure. "Worthwhile" is definitely the key
question we need to decide. For me, it seems worth it in theory
because it seems like it shouldn't be too hard to convert. The only
way to find out is to attempt to convert one of my most date-intensive
applications (including database) see how it goes, see what issues
arise.
The simplicity of the large integer utcMicroseconds representation trumps all the nonsense of breaking it down into sub components. In any case the 32-bit VM is communicating time up to the image as 64-bit large integer microseconds anyway. Not decomposing gives much faster instantiation, and very simple arithmetic (simply compare the utcMicroseconds).
I've been getting a good look at UTCDateAndTime and emailing Dave for
the last few days. I like it for 64-bit and agree with your
sentiments except for the old way being "nonsense". It was made in
the early 2000's when 32-bit was all we had. It beat a LargeInteger
based competitor I tried to use for a while so I switched.

Dave has been patient helping me get the Ma Serializer upgraded. I'm
trying to find a way to preserve forward-compatibility in legacy
systems, otherwise it means having to shut everything down and upgrade
every client image. Simultaneously. Big bang style.
Note that it would be trivial to extend the representation with the decomposed elements, an d these could be nil initially and instantiated on demand. Forcing the large integer arithmetic to decompose on every instantiation would kill any performance advantage one might expect to get from using immediate instead of there large integer. And of course in 64-bits, utcMicroseconds is an immediate anyway.
That is an **amazing idea**! The code should decompose the new
utcMicroseconds to the old smaller values on-demand but also the
reverse. For UTC clients reading legacy data, the 'utcMicroseconds'
variable will be mapped to nil upon materialization, after which the
code would lazily calculate it on-demand. Brilliant!

Will definitely have to look

Chris Cunningham
2018-11-20 19:22:49 UTC
Permalink
HI.
Post by Chris Muller
Post by Chris Muller
Hi Dave, I took a look at Chronology-Core-dtl.30, it looks like a good
change.
Post by Chris Muller
Now we just need something to obtain a canonical date from
DateAndTime's similar to Chronology's #beCanonical. BUT, given the
confusion we've had about #asDate in the past, what if we changed
#asDate to do the "expected" behavior -- answer the canonical version
--, and added, say, #asLocalDate to preserve the not-so-common
version?
Hi Chris,
That sounds reasonable enough to me, although I would defer to you and
others as to what is the preferred default because I really don't have
any applications that would be impacted one way or the other.
I think I would prefer to have DateAndTime now asDate (the asDate part)
return a canonical date - one with the starting time having no offset at
all. While I have had cases to use the local date, they are actually rare
and far between. On the other hand, wanting a canonical date is very
common, and apparently expected by most other users.
I definitely approve of adding a #asLocalDate for those cases where I want
the local date.

Along these lines, I think we should make the default #asMonth, #asYear,
#asWeek also return canonical instances. These make even less sense to
have in the 'local' time.
That said, the default Duration probably doesn't deserve the same canonical
treatment - if I want 2-1/2 hours starting now, I definitely want it in the
same timezone!

-cbc

ps - I strongly vote for integrating UTCDateAndTime into Trunk, having just
been bitten by the daylight savings again - my long running image never
switched over to the new offsets, since that apparently only happens when
you restart the image. So, wrong offset for a good week before I noticed.
Post by Chris Muller
What's your feeling overall regarding UTCDateAndTime? Does it seem like
a worthwhile change from your point of view? And to my main worry, is
there anything about the different layout of instance variables that is
likely to cause problems for Magma, or for applications that use other
databases for persistence? I think that it will be OK, but it's better
to ask now than to be sorry later.
Thanks,
Dave
Post by Chris Muller
- Chris
Post by David T. Lewis
Post by David T. Lewis
Hi Chris,
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it
left me
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
some time to actually start looking at it today.
A cursory look reveals that, at least functionally, everything
appears
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
to be essentially the same as original Chronology except the
on-going
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
What are the point-in-time endpoints of a timespan
identified by a particular "Date"?
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
or 2) "global" e.g., a period that begins from 12am to
11:59:59.999999 of UTC (offset 0).
We already know that original Chronlology supports ability for
applications to represent their Dates either way. What about
DateAndTime now asDate = Date today
I get "true", even though MY 22-Oct-2018 begins and ends at a
different point-in-time than those in the UTC timezone..
I realize most applications want a canonical representation of
Dates,
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
but where does this leave the timespan use-cases? Are they even
possible at all? What if I truly need the same local
representation
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
Post by Chris Muller
of 22-Oct-2018 that everyone else recognizes in my local area?
I think the difference is when we get to Date class>>starting: which
sets the starting point to be aDateAndTime midnight.
DateAndTime now asDate start ==> 2018-10-22T00:00:00-04:00
DateAndTime now asDate start ==> 2018-10-22T00:00:00+00:00
So I think you are right. Given that a date is modeled as a
duration, the
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
start time of the date should align with the timezone of the
DateAndTime
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
from which it was created.
DateAndTime>>midnight
"Answer a DateAndTime starting at midnight of the same
timezone offset as the receiver."
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
^ self class basicNew
setJdn: self julianDayNumber
seconds: 0
nano: 0
offset: (Duration seconds: localOffsetSeconds)
But fixing that triggers one other test failure so I need to sort
that
Post by Chris Muller
Post by David T. Lewis
Post by David T. Lewis
out before posting anything.
Thanks for looking at this, much appreciated.
I added a test to cover this case in Chronology-Tests in trunk, and a
fix in the UTCDateAndTime repository in Chronology-Core-dtl.30.
Dave
Sean P. DeNigris
2018-10-23 15:17:44 UTC
Permalink
Post by Chris Muller
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
Not to reignite the long and numerous debates on the topic, but IHMO the
seemingly unresolvable (in-practice) problem with appending offsets to
historical dates is the offset-changing effect of daylight savings time.
What we want to know is "the offset of my local time on the date in
question", but all we know is "the local offset today". This leads to subtle
bugs like:
"executed just before DST"
timestamp := '1/1/2016' asDate start.
"executed just after"
timestamp = '1/1/2016' asDate start "false - WTH!"

The only solution seems to be more seriously modeling timezones w.r.t. all
the dates effecting them via an external authority. This is a nasty bug and
again IMHO is probably better not to provide this kind of behavior at all
rather than provide it in a subtly wrong way.



-----
Cheers,
Sean
--
Sent from: http://forum.world.st/
Chris Muller
2018-10-23 20:57:46 UTC
Permalink
Post by Sean P. DeNigris
Post by Chris Muller
1) "local" e.g., a period that begins from 12am to
11:59:59.999999 of my LOCAL time,
Not to reignite the long and numerous debates on the topic, but IHMO the
seemingly unresolvable (in-practice) problem with appending offsets to
historical dates is the offset-changing effect of daylight savings time.
What we want to know is "the offset of my local time on the date in
question", but all we know is "the local offset today". This leads to subtle
"executed just before DST"
timestamp := '1/1/2016' asDate start.
"executed just after"
timestamp = '1/1/2016' asDate start "false - WTH!"
The above example was fixed in Squeak in 2011 by introducing Timespan
class>>#defaultOffset (which at the time was Duration zero, but has
since been changed to nil) for Timespan instances (incl. Date, Month,
Year), and letting all future created instances "inherit" their
TZ-context from whatever other source / calculation that created them.
Since Timespans (incl. Date, Month, Year) created via the class
constructors have no such TZ-context, they assume the #defaultOffset,
and any instances that emerge from calculations, etc. from those, will
assume that same no-TZ context, too. Kinda like a virus...
Post by Sean P. DeNigris
The only solution seems to be more seriously modeling timezones w.r.t. all
the dates effecting them via an external authority. This is a nasty bug and
again IMHO is probably better not to provide this kind of behavior at all
rather than provide it in a subtly wrong way.
I agree with your sentiments about bug types, but I don't think this
case is that serious. Yes, it can and has been confusing for
developers new to Squeak before they understand that Dates's can be
_local_, and indeed do inherit that property from
DateAndTime>>#asDate. But that's pretty much the only confusing one
and my hope is that by adding #asCanonicalDate, it will be "seen" by
them and help accelerate that learning curve. Once they know, it's
pretty easy to know, in any code context, where a #beCanonical
conversi
David T. Lewis
2018-11-08 02:40:29 UTC
Permalink
Post by Chris Muller
Hi Dave,
Thanks for making UTCDateAndTime available on SqueakMap that "just
works" -- not having to go research how to properly install it left me
some time to actually start looking at it today.
The SqueakMap entry for UTCDateAndTime has started to fail due to a bug
in Installer. A proposed fix is in the inbox as Installer-Core-dtl.425.

The Installer script started failing when I later published
'Chronology-Core-dtl.30' to the repository, which Installer is now
confusing with the 'Chronology-Core-dtl.3' version that it should
actually be loading. The fix in the inbox, or some similar fix, will
make the SqueakMap entry for UTCDateAndTime work properly again.
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