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

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.