How to find out the current time in DST or not?

Question

How to find out if the current time is in a certain city in the world in the summer or not? Is there a PHP function?

Answer 1

I'm not a PHP person, but it seems like it's possible - but inconvenient.

Given a DateTimeZone , you can find the current offset and set of transitions. Therefore, if you ask for transitions to "now", you can find out information about the current part of the time zone. A slightly modified example from the docs :

$theTime = time(); # specific date/time we're checking, in epoch seconds. 

$tz = new DateTimeZone('America/Los_Angeles'); 
$transition = $tz->getTransitions($theTime,$theTime); 

# only one array should be returned into $transition.
$dst = $transition[0]['isdst']; 

Answer 2

Well, it looks like I isdstshould be doing the right thing, but I have been bitten before, so here is a quick port of my C ++ timezone code for PHP:

// given a timezone and a timestamp
// return true if timezone has DST at timestamp, false otherwise
// timezone defaults to current timezone
// timestamp defaults to now
function is_dst($timezone = null, $time = null) {
    $oldtimezone = date_default_timezone_get();
    if (isset($timezone)) {
        date_default_timezone_set($timezone);
    }

    if (!isset($time)) {
        $time = time();
    }

    $tm = localtime($time, true);
    $isdst = $tm['tm_isdst'];
    $offset = 0;
    //$dsttime = mktime_array($tm);
    //echo strftime("%c", $dsttime);

    $tm['tm_isdst'] = 0;
    $nondsttime = mktime_array($tm);
    $offset = $nondsttime - $time;

    date_default_timezone_set($oldtimezone);

    return $offset != 0;
}

function mktime_array($tm) {
    return mktime($tm['tm_hour'], $tm['tm_min'], $tm['tm_sec'], $tm['tm_mon']+1, $tm['tm_mday'], $tm['tm_year']+1900, isset($tm['tm_isdst'])? $tm['tm_isdst']: -1);
}

, :

foreach (array(null, "Australia/Sydney", "UTC", "America/Los_Angeles") as $tz) {
    $isdst = is_dst($tz);
    if (isset($tz)) {
        echo $tz;
    }
    else {
        echo "current timezone";
    }
    echo " ";
    if ($isdst) {
        echo "has daylight savings now\n";
    }
    else {
        echo "has standard time now\n";
    }
}

// tests based on known transitions for Sydney (AEST)
foreach (array(null, "2011-04-03 01:59:00", "2011-04-03 02:00:00", "2011-10-02 01:59:00", "2011-10-02 03:00:00") as $timestr) {
    $tz = "Australia/Sydney";
    if (isset($timestr)) {
        $tm = strptime($timestr, "%Y-%m-%d %H:%M:%S");
        $time = mktime_array($tm);
    }
    else {
        $time = time();
    }
    $isdst = is_dst($tz, $time);
    if (isset($tz)) {
        echo $tz;
    }
    else {
        echo "current timezone";
    }
    echo " ";
    if ($isdst) {
        echo "has daylight savings at $timestr\n";
    }
    else {
        echo "has standard time at $timestr\n";
    }
}

:

current timezone has daylight savings now
Australia/Sydney has daylight savings now
UTC has standard time now
America/Los_Angeles has standard time now
Australia/Sydney has daylight savings at 
Australia/Sydney has daylight savings at 2011-04-03 01:59:00
Australia/Sydney has standard time at 2011-04-03 02:00:00
Australia/Sydney has standard time at 2011-10-02 01:59:00
Australia/Sydney has daylight savings at 2011-10-02 03:00:00