Timezone with Daylight Savings Detection

[username]

Hi all, I have been searching for a reliable script that calculates a users timezone offset, i.e GMT + 10. I want the script to also handle daylight savings periods also.

I found this script which seems to work, but I am not currently in DST, so am wondering if anyone is, or has a bullet proof script for calculating this that also handles half hours etc?

<script type="text/javascript">
function checkTimeZone() {
var rightNow = new Date();
var date1 = new Date(rightNow.getFullYear(), 5, 1, 0, 0, 0, 0);
var date2 = new Date(rightNow.getFullYear(), 6, 1, 0, 0, 0, 0);
var temp = date1.toGMTString();
var date3 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
var temp = date2.toGMTString();
var date4 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
var hoursDiffStdTime = (date1 - date3) / (1000 * 60 * 60);
var hoursDiffDaylightTime = (date2 - date4) / (1000 * 60 * 60);
if (hoursDiffDaylightTime == hoursDiffStdTime) {
document.write("Time zone is GMT " + hoursDiffStdTime + ".\nDaylight Saving Time is NOT observed here.");
} else {
document.write("Time zone is GMT " + hoursDiffStdTime + ".\nDaylight Saving Time is observed here.");
}
}
</script>

[g1smd]

One note. Use UTC in place of GMT. GMT was deprecated in 1971.

[username]

Ok, cool. So obviously this script has some issues. Does any have a script using UTC that handles DST and returns the offset. i.e. 10 hours

[g1smd]

It's just a text change in the visual printed messages.

The variable name inside PHP is still called GMTstring.

[penders]

...Use UTC in place of GMT. ...The variable name inside PHP is still called GMTstring.

In JavaScript, the toGMTString() method is deprecated in favour of toUTCString() [w3schools.com]. The output, however, still contains the text 'GMT'.

JavaScript Date Object [w3schools.com]

[penders]

Your script doesn't work for me. It correctly picks up the timezone offset (but that's the easy part... Date.timeZoneOffset() would do the same), but it states that DST is "NOT observed here", but it is (and I'm currently on DST here in the UK).

I think the script should state "DST is observed here" if your region switches to DST at some point, not whether you are currently on DST or not.

HOWEVER, and this is the problem, the script assumes DST switches at some point between June 1 and July 1 (date1 and date2 respt). What region is that? In the UK DST starts at the end of March. The script needs to test a date on either side of the switch, apart from that the dates really don't matter (I don't think).

If I change:

var date1 = new Date(rightNow.getFullYear(), 5, 1, 0, 0, 0, 0);

To

var date1 = new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0); // 0 - January

This then works for me. I think it will also work for the original region. Does it work for everyone? With the dates 6 months apart, I think it stands a good chance. But elsewhere in the world DST could start at anytime and last for longer than 6 months?! In the UK DST lasts for 7 months.

With these last points in mind I have my doubts whether you can write a compact JavaScript function that is able to test DST that will work worldwide, without looking up some local knowledge? How variable is DST around the world?

Actually, I wonder... if you know the timezone offset, can you make an intelligent assumption as to when the DST might be? eg. If offset is 0 or 1 then DST changes in March, then work backwards... If offset is -1, assume DST changes in February... etc?

[g1smd]

DST implementation is very variable around the world.

When the US changes over there is a one-hour delay between each time zone. A similar effect is seen in Europe on a completely different date.

The dates around the world span several months.

More importantly the Southern hemisphere uses DST from roughly September to March while the Northern hemisphere uses it from roughly March to September.

Many places use DST for 7 months at a time.

The changes for the last half of 2011 are listed here: [timeanddate.com...]