> On 9 Apr 2018, at 15:19, Sean P. DeNigris <s...@clipperadams.com> wrote:
> 
> Max Leske wrote
>> Assuming UTC is probably just as wrong as assuming the local time zone.
> 
> I'll modify your statement slightly. "Assuming UTC is */almost/* as wrong as
> assuming the local time zone."
> 
> I've never seemed to be able to drive the essential point home when these
> discussions have come up. Beyond all the wider issues, there is a bug, plain
> and simple: The offset of `'1/1/1990' asDate`, considering that you mean
> local time, is still not guaranteed to be the current local offset of the
> image, which is how we set it by default. That only makes sense if the
> historical date in question was in the same state of DST as the current
> image. For example, the historical date above is in winter, so if I eval
> that code in summer, it will /always/ give the objectively wrong offset.
> 
> What I'm proposing is not a cure all, but a slightly-better way that fixes
> this bug by giving users consistent behavior that they may not want instead
> of inconsistent and often wrong behavior that they may not want.

Sean,

You are right, the current system cannot be fixed. It only knows about the 
current timezone's offset (via the OS), not about historical offsets. And it 
wrongly uses that offset because it does not know better.

Neither 

  '1/1/1990' asDate.

nor

  Date today.

can work without the context of a precise timezone. It is even relatively 
pointless to remember offsets without remembering timezones. You simply need a 
precise reference into the transitions database.

New York is 5 hours behind UTC in winter.

Question 1: When (in absolute UTC time) was the beginning of the 1st day of 
January in 1990 in New York's local time, when we express the date in UTC ?

(ZTimezone id: 'America/New_York') gmtToLocal: (ZTimestamp @ '1990/01/01'). 

  => "1989-12-31T19:00:00Z"

So when the UTC day of January 1st 1990 starts, New York local time is still 5 
hours behind. 

Question 2: When (in absolute UTC time) was the beginning of the 1st day of 
January in 1990 in UTC time, when we express the date locally ?

(ZTimezone id: 'America/New_York') localToGmt: (ZTimestamp @ '1990/01/01').

  => "1990-01-01T05:00:00Z"

So when the New York day of January 1st 1990 starts, UTC time is already 5 
hours ahead.

Note that the question 'When does January 1st 1990 start in any timezone, when 
expressed in that timezone, is of course a constant, midnight'.


I think that making Date always UTC will probably not help, because you will 
want to be able to move between timezones. I guess the only solution is to add 
a class like ZTimezone (which has no dependencies).


Sven

> -----
> Cheers,
> Sean
> --
> Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html
> 


Reply via email to