Timestamps, Timezones, Offsets, DST, etc. in PHP

[lisa71283]

Hi all,

 

 

I am currently developing a rather intensive, non-local PHP application, and have reached the point at which localization issues must be addressed. I have never been faced with doing anything global before, so my experience with the concept of time in PHP has simply been operating everything (including DB-stored timestamps) as the local time(), and allowing users to offset an integral number of hours from that. This solution is not helpful to international visitors, and also not helpful to those living in the US who do not observe daylight savings time.

 

I have committed to using UTC integer timestamps for all internal representation, and apply a localization filter to the output, based on stored profile settings.

 

The problem comes in when daylight savings time is applied to the mix. What is the best way to apply DST rules (based on a user's chosen [international] location) to a UTC timestamp when processing localization logic? Does PHP provide a native functionality for doing this, or must I research DST in all of the world's locales?

 

PEAR's DATE class is not an option. Even though this application will be running an a server supporting opcode caching, the PEAR library is horrendously bloated for such a simple task. I am not going to include 676KB of code just to process timezone and DST rules.

 

Most of the code snippets I have found across the Internet include no mention of the conceptual basis for the code, and simply instruct the user to C&P x if they would like to accomplish y. Based on my testing, much of this code is self-contradictory. I can't rely on something which may break during a DST shift, or in a timezone the author never sees. From my experience, date/time errors are particularly insidious, hard to notice, and very difficult to correct.

 

What is the best way to use PHP to convert from the application's internal UTC timekeeping to something localized for a user? I don't know that I can simply date_default_timezone_set(), as I will be operating on timestamps pulled from databases and subsequently manipulated by application logic?

 

Knowing that everything internal is going to be UTC, how would I go about soliciting a user's offset, DST (not just American rules) preferences, and other applicable information, then applying that to the application's UTC for eventual output to the user?

 

Does anyone have a good conceptual tutorial covering international date/time handling in PHP? Functional code snippets are great when you know they work, but I think that it is also to understand the "why" behind the logic, especially if the code hasn't gone through extensive, community-based QC.

[The Little Guy]

Have a look at this: http://phpsnips.com/snippet.php?id=51