How to tell if a timezone observes daylight saving at any time of the year?

Question

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?

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Edit to clarify:

• Brisbane, Australia does not observe daylight savings at any time of the year. All year around, it is GMT+10.
• Sydney, Australia does, from October to March when it changes from GMT+10 to GMT+11.

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

Answer 1

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.