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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);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 Pacific and Eastern time are observing DST, and the calculation is correct again (putting California at seven hours behind UTC).
Recipe 3.12 for dealing with time zones; documentation on putenv( ) at http://www.php.net/putenv, localtime( ) at http://www.php.net/localtime, gmdate( ) at http://www.php.net/gmdate, and gmstrftime( ) at http://www.php.net/gmstrftime; a detailed presentation on DST is at http://webexhibits.org/daylightsaving/.
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| 3.12. Calculating Time with Time Zones |  | 3.14. Generating a High-Precision Time |
Copyright © 2003 O'Reilly & Associates. All rights reserved.