How to tell if a timezone observes daylight saving at any time of the year?
In PHP, you can tell if a given date is during the Daylight Savings Time period by using something like this:
$isDST = date("I", $myDate); // 1 or 0
The problem is that this only tells you whether that one point in time is in DST. Is there a reliable way to check whether DST is in effect at any time in that timezone?
Edit to clarify:
I'm wondering if there would be some existing method, or a way to implement a method which works as such:
timezoneDoesDST('Australia/Brisbane'); // false
timezoneDoesDST('Australia/Sydney'); // true
I've found a method which works using PHP's DateTimezone class (PHP 5.2+)
function timezoneDoesDST($tzId) {
$tz = new DateTimeZone($tzId);
$trans = $tz->getTransitions();
return ((count($trans) && $trans[count($trans) - 1]['ts'] > time()));
}
or, if you're running PHP 5.3+
function timezoneDoesDST($tzId) {
$tz = new DateTimeZone($tzId);
return count($tz->getTransitions(time())) > 0;
}
The getTransitions() function gives you information about each time the offset changes for a timezone. This includes historical data (Brisbane had daylight savings in 1916.. who knew?), so this function checks if there's an offset change in the future or not.
Actually nickf method didn't works for me so I reworked it a little ...
/**
* Finds wherever a TZ is experimenting dst or not
* @author hertzel Armengol <emudojo @ gmail.com>
* @params string TimeZone -> US/Pacific for example
*
*/
function timezoneExhibitsDST($tzId) {
$tz = new DateTimeZone($tzId);
$date = new DateTime("now",$tz);
$trans = $tz->getTransitions();
foreach ($trans as $k => $t)
if ($t["ts"] > $date->format('U')) {
return $trans[$k-1]['isdst'];
}
}
// Usage
var_dump(timezoneExhibitsDST("US/Pacific")); --> prints false
var_dump(timezoneExhibitsDST("Europe/London")); --> prints false
var_dump(timezoneExhibitsDST("America/Chicago")); --> prints false
same function call will return true in 1 month (March) hope it helps
DateTimeZone::getTransitions might help.
You could probably wing it:
$hasDst = date("I", strtotime('June 1')) !== date("I", strtotime('Jan 1'));
Otherwise you'd need to parse the text-based zoneinfo data files.
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