3.12. Accounting for Daylight Saving Time
Problem
You need to make sure your time calculations properly consider DST.
Solution
The zoneinfo library calculates the effects of
DST properly. If you are using a Unix-based system, take advantage of
zoneinfo with
putenv( )
:
putenv('TZ=MST7MDT'); print strftime('%c');
If you can’t use zoneinfo, you
can modify hardcoded time-zone offsets based on whether the local time
zone is currently observing DST. Use localtime( )
to determine the current DST
observance status:
// Find the current UTC time $now = time(); // California is 8 hours behind UTC $now -= 8 * 3600; // Is it DST? $ar = localtime($now,true); if ($ar['tm_isdst']) { $now += 3600; } // Use gmdate() or gmstrftime() to print California-appropriate time print gmstrftime('%c',$now);
Discussion
Altering an
epoch timestamp by the amount of a time
zone’s offset from UTC and then using
gmdate( )
or gmstrftime( )
to print out time zone-appropriate functions is flexible — it works from any time zone — but the DST calculations are slightly inaccurate. For the brief intervals when the server’s DST status is different from the target time zone’s, the results are incorrect. For example, at 3:30 A.M. EDT on the first Sunday in April (after the switch to DST), it’s still before the switch (11:30 P.M.) in the Pacific time zone. A server in Eastern time using this method calculates California time to be seven hours behind UTC, whereas it’s actually eight hours. At 6:00 A.M. EDT (3:00 A.M. PDT), both ...
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