Recipe 3.13 Accounting for Daylight Saving Time
3.13.1 Problem
You need to make sure your time calculations
properly consider DST.
3.13.2 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);
3.13.3 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 Pacific and Eastern time are observing DST, and the
calculation is correct again (putting California at seven hours
behind UTC).
3.13.4 See Also
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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