I  think #= is a bad selector for Date and should be avoided when determining
whether something happens on a date, or whether two dates are the same.   We
all know March 24th in London covers a different 24 hours than March 24th in
Hawaii but Date>>#= does not.  



I think whats needed are more descriptive selectors like


Date>>#isSameOnDateAs: aDateOrDateAndTime
Date>>#overlapsWithDate: aDate
DateAndTime>>#occursOnDate: aDate
DateAndTime>>#sameHMSButDifferentUTCIn: aTimeZoneKey
DateAndTime>>#sameUTCButDifferentHMSIn: aTimeZoneKey

and change Date>>#= to #shouldNotImplement.


FWIW I also don't like #offset: as before you send it you know the timezone
and after you may let that knowledge be forgotten. Real offsets can change
as laws change.



I think people are aware of this but if you have need for comparing dates &
times then you must use a library that accesses the regularly updated Olson
timezone database on your system and classes that respect time zones.  Time
zones are political, and legal definitions of offsets can change hours
before the DST transition dates & times.  


I don't think it matters which default timezone you pick for the image if
you're not going to take them into account when doing comparisons.  


Unfortunately there isn't a way to avoid this complexity until DST goes
away.  


There's certainly flaws to how we currently do it and I think
TimeZoneDatabase and Chronos make good attempts to fix it.  I haven't looked
at Chalten but would guess its good too.   








Sean P. DeNigris wrote
> I was bitten by this very annoying bug again. As most of us probably know
> due
> to the steady stream of confused ML posts in the past, the bug in summary
> is
> that we have an incomplete timezone implementation that doesn't properly
> take into account historical DST changes. This flares up without warning
> especially when DST toggles. I created a wiki page to document the
> situation: https://github.com/seandenigris/pharo/wiki/Time-Zone-Fiasco
> 
> Here's an example blowup: at 11:59pm before DST changes, eval aDate :=
> '1/1/1901' asDate. Now, wait two minutes and at 12:01am eval self assert:
> '1/1/1901' asDate = aDate and… whammo, an exception! The "different"
> offsets
> render equal dates unequal depending on when the objects were created.
> 
> The more I think about it, the more I think that we should just assume UTC
> for all Date[AndTime]s that don't explicitly specify an offset, rather
> than
> pretend to set an offset which is only sometimes correct. More advanced
> users can use one of the available libraries to get full timezone support.
> What do you think?
> 
> 
> 
> -----
> Cheers,
> Sean
> --
> Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html





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