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Timezone/DST handling in Javascript

Timezone/DST handling in Javascript

December 15th, 2008

Mark was inspired on Friday and wrote this lovely piece of code for detecting whether daylight savings time is enabled, which he is particularly proud of.

I have found it tends to be quite difficult to define what people mean when they specify a date and time.

* Do they mean GMT time? Unless they are living within the GMT timezone, more likely is that they mean their local time.
* What is the effect of daylight saving? It may or may not be in effect at a particular point in time, and possibly not even being used in the local timezone.

Javascript gives us the Date.getOffsetTimezone however AFAIK there is nothing similar for DST. The following Date protoype extension (based on the technique by Josh Frasher) detects whether DST is enabled:


Date.prototype.getDST() = function() {
var now_local = this;
var jan_local = new Date(now_local.getFullYear(), 0, 1, 0, 0, 0, 0, 0);
var jun_local = new Date(now_local.getFullYear(), 5, 1, 0, 0, 0, 0, 0);
var now_utc = new Date.UTC(now_local.getFullYear(), now_local.getMonth(), now_local.getDate(), now_local.getHours(), now_local.getMinutes(), now_local.getSeconds(), now_local.getMilliseconds());
var jan_utc = new Date.UTC(jan_local.getFullYear(), jan_local.getMonth(), jan_local.getDate(), jan_local.getHours(), jan_local.getMinutes(), jan_local.getSeconds()), jun_local.getMilliseconds();
var jun_utc = new Date.UTC(jun_local.getFullYear(), jun_local.getMonth(), jun_local.getDate(), jun_local.getHours(), jun_local.getMinutes(), jun_local.getSeconds(), jan_local.getMilliseconds());
var now_diff = Math.round((now_utc - now_local.getTime()) / (1000 * 3600));
var jan_diff = Math.round((jan_utc - jan_local.getTime()) / (1000 * 3600));
var jun_diff = Math.round((jun_utc - jun_local.getTime()) / (1000 * 3600));
return jan_diff != jun_diff && now_diff == Math.max(jan_diff, jun_diff);
}


Feel free to use/modify this code as you see fit.

Update: l8nite (see comment #1 below) pointed out a few errors when the times are right on the cusp of a DST update. This was down to the Date object doing internal conversions based on the user’s timezone (thus messing up the UTC times being passed in).

Update 2: Unfortunately this code relies on the operating system’s timezone settings and whether DST patches (DST changes all the time!) have been applied. Having done some testing in Win XP SP3 it seems that DST settings for the U.S. are a bit messed up.

Update 3: Updated to use UTC method from the class rather than a new object and added milliseconds (thanks l8nite comment #6)

Update 4: Using Math.round rather than parseInt to avoid rounding issues (thanks Eyal comment #7)