Javascript/PHP and timezones

Question

I\'d like to be able to guess the user\'s timezone offset and whether or not daylight savings is being applied. Currently, the most definitive code that I\'ve found for this

Answer 1

If this is a poll, then "b" is my vote.

Firstly, all times should be stored in UTC. This is dogma. It's very sensible dogma, the kind about which you end up saying "I sure wish I had followed that," after your project gets more complicated. Among other things, it is the only unambiguous, consistent way to store all points in time. Any timezone with daylight savings time has ambiguous time references around the switch (1:30 am usually happens twice, for example). Also, when converting between timezones, most of the time you end up using UTC as an intermediary anyway.

Second of all, you have to decide whether your site is international or not. If not, then you make rules for the six U.S. time zones and end it. With three browsers reporting timestamps in their own whacky ways, that's still only 18 cases, and it should be possible to handle them. Anything beyond that, and you should assume that it requires an advanced degree to anticipate daylight savings time differences. The times switch on different days in different places.

Your biggest problem with b is that, if this is a calendar-like application, scheduling will still be an issue if you can't accurately determine what time zone someone is in. For example, suppose it's February. No one is on DST. Someone schedules something for 6pm (local) on May 5. You see the offset is UTC-4. How do you know whether that person is in New Brunswick, which observes DST (in which case the time meant is 2100 UTC), or Puerto Rico, which does not (in which case the time meant is 2200 UTC? It's a tricky question. This post may have some help.