[Tickets #13947] Re: Calendar breaks on DST start date

[noreply at bugs.horde.org]

DO NOT REPLY TO THIS MESSAGE. THIS EMAIL ADDRESS IS NOT MONITORED.

Ticket URL: https://bugs.horde.org/ticket/13947
------------------------------------------------------------------------------
  Ticket             | 13947
  Updated By         | jsveiga at it.eng.br
  Summary            | Calendar breaks on DST start date
  Queue              | Kronolith
  Version            | 4.2.2
  Type               | Bug
  State              | Feedback
  Priority           | 2. Medium
  Milestone          |
  Patch              |
  Owners             |
------------------------------------------------------------------------------


jsveiga at it.eng.br (2015-04-13 17:50) wrote:

> It's a bug with the DateJS library then.

(sorry if I'm flooding this with comments; let me know and I'll be quiet)

It is hard to call that a bug, because there is no "right" way to do  
it. Just adding 1 to the month day number would be wrong, because  
"Date" is actually a "DateTime", so the time has to be taken into  
account too.

If you think about it, 00:00 - 00:59 October 18th 2015 does not exist  
in my timezone because at 23:59:59.99 on the 17th you set your clock  
to 01:00:00.00 (and each DST-using timezone has such gap).

So adding 1 day to "Oct 17th 00:00" cannot result in "Oct 18th 00:00"  
as that is an invalid datetime in this timezone. IMO, the "best"  
result would be that the .add or .next methods returned an "invalid  
date", and left it to the calling program to decide what to do.  
Instead, each implementation seems to "decide" what to return (in some  
Android cases, it's 31/12/1969 00:00!).

So when we are just interested in the Date and not Time, it is much  
safer to set the time to something far from 00:00 (some timezones  
switch at 01:00) before doing date math.