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PHP daylight saving time detection

Asked 2011-05-27T16:35:44.103
35

I need to send an email to users based wherever in the world at 9:00 am local time. The server is in the UK. What I can do is set up a time difference between each user and the server's time, which would then perfectly work if DST didn't exist.

Here's an example to illustrate it:

  • John works in New York, -5 hours from the server (UK) time
  • Richard works in London, UK, so 0 hour difference with the server.

When the server goes from GMT to GMT +1 (BST) at 2:00am on a certain Sunday, this means that John now has a -6H time difference now.

This scenario I can still handle by updating all the users outside the server's local time, but once I've moved forward/backward the time of all the other users, I still need a way to detect when (time and date) the users living outside the UK will (or will not) change their local time to a probable DST one.

I need a PHP method to know/detect when other parts of the world will enter/exit DST.

php dst gmt binary-search-tree
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1 Answer

29

As Jimmy points out you can use timezone transitions, but this is not available on PHP <5.3. as dateTimeZone() is PHP>=5.2.2 but getTransitions() with arguments is not! In that case here is a function that can give you timezone data, including whether in DST or not.

function timezonez($timezone = 'Europe/London'){
    $tz = new DateTimeZone($timezone);
    $transitions = $tz->getTransitions();
    if (is_array($transitions)){
        foreach ($transitions as $k => $t){
            // look for current year
            if (substr($t['time'],0,4) == date('Y')){
                $trans = $t;
                break;
            }
        }
    }
    return (isset($trans)) ? $trans : false;
}

Having said that, there is a simpler method using date() if you just need to know whether a timezone is in DST. For example if you want to know if UK is in DST you can do this:

date_default_timezone_set('Europe/London');
$bool = date('I'); // this will be 1 in DST or else 0

... or supply a timestamp as a second arg to date() if you want to specify a datetime other than your current server time.

answered 2012-06-14T12:02:17.687

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