How to check if DST (Daylight Saving Time) is in effect, and if so, the offset?

This code uses the fact that getTimezoneOffset returns a greater value during Standard Time versus Daylight Saving Time (DST). Thus it determines the expected output during Standard Time, and it compares whether the output of the given date the same (Standard) or less (DST).

Note that getTimezoneOffset returns positive numbers of minutes for zones west of UTC, which are usually stated as negative hours (since they're "behind" UTC). For example, Los Angeles is UTC–8h Standard, UTC-7h DST. getTimezoneOffset returns 480 (positive 480 minutes) in December (winter, Standard Time), rather than -480. It returns negative numbers for the Eastern Hemisphere (such -600 for Sydney in winter, despite this being "ahead" (UTC+10h).

Date.prototype.stdTimezoneOffset = function () {
    var jan = new Date(this.getFullYear(), 0, 1);
    var jul = new Date(this.getFullYear(), 6, 1);
    return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset());
}

Date.prototype.isDstObserved = function () {
    return this.getTimezoneOffset() < this.stdTimezoneOffset();
}

var today = new Date();
if (today.isDstObserved()) { 
    alert ("Daylight saving time!");
}

This answer is quite similar to the accepted answer, but doesn't override the Date prototype, and only uses one function call to check if Daylight Savings Time is in effect, rather than two.

The idea is that, since no country observes DST that lasts for seven months^[1], in an area that observes DST the offset from UTC time in January will be different to the one in July.

While Daylight Savings Time moves clocks forwards, JavaScript always returns a greater value during Standard Time. Therefore, getting the minimum offset between January and July will get the timezone offset during DST.

We then check if the dates timezone is equal to that minimum value. If it is, then we are in DST; otherwise we are not.

The following function uses this algorithm. It takes a date object, d, and returns true if daylight savings time is in effect for that date, and false if it is not:

function isDST(d) {
    let jan = new Date(d.getFullYear(), 0, 1).getTimezoneOffset();
    let jul = new Date(d.getFullYear(), 6, 1).getTimezoneOffset();
    return Math.max(jan, jul) !== d.getTimezoneOffset();    
}

Create two dates: one in June, one in January. Compare their getTimezoneOffset() values.

• if January offset > June offset, client is in northern hemisphere
• if January offset < June offset, client is in southern hemisphere
• if no difference, client timezone does not observe DST

Now check getTimezoneOffset() of the current date.

• if equal to June, northern hemisphere, then current time zone is DST (+1 hour)
• if equal to January, southern hemisphere, then current time zone is DST (+1 hour)

I was faced with this same problem today but since our daylight saving starts and stops at differing times from the USA (at least from my understanding), I used a slightly different route..

var arr = [];
for (var i = 0; i < 365; i++) {
 var d = new Date();
 d.setDate(i);
 newoffset = d.getTimezoneOffset();
 arr.push(newoffset);
}
DST = Math.min.apply(null, arr);
nonDST = Math.max.apply(null, arr);

Then you simply compare the current timezone offset with DST and nonDST to see which one matches.