How to Check If Dst (Daylight Saving Time) Is in Effect, and If So, the Offset

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!");
}

javascript - check if daylight time is in effect for different timezone

If you use moment.js and it's companion timezome script, they make checking for DST pretty easy:

Check out their docs for isDST()

Just for grins, here's a fiddle testing each timezone

if( moment.tz("America/New_York").isDST() ){    $('#test').append('NY is currently in DST');}else{    $('#test').append('NY is NOT currently in DST');}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script><script src="http://cdnjs.cloudflare.com/ajax/libs/moment.js/2.8.3/moment.min.js"></script><script src="http://momentjs.com/downloads/moment-timezone-with-data.min.js"></script>

<div id="test"></div>

Check if Daylight Saving Time (DST) is in effect with Julia

I wouldn't use Libc at all here; the Julia standard library Dates in conjunction with TimeZones package is far more functional and featureful. First, parse your date as a timezoneless Date:

julia> using Dates, TimeZones

julia> format = dateformat"Y/m/d H:M:S"
dateformat"Y/m/d H:M:S"

julia> dt = DateTime("2020/10/26 11:08", format)
2020-10-26T11:08:00

Then append the local time zone:

julia> zdt = ZonedDateTime(dt, localzone())
2020-10-26T11:08:00-04:00

Finally, you can ask if there will be any future transitions — and what they are:

julia> show_next_transition(zdt)
Transition Date:   2020-11-01
Local Time Change: 02:00 → 01:00 (Backward)
Offset Change:     UTC-5/+1 → UTC-5/+0
Transition From:   2020-11-01T01:59:59.999-04:00 (EDT)
Transition To:     2020-11-01T01:00:00.000-05:00 (EST)

Note that the concept of "Daylight Savings" isn't explicitly encoded into a timezone — there simply exist variable timezones that change which constant offset they're in at given times. You can see in the above output that I'm currently in UTC-4, but will be switching to UTC-5 on Sunday.

So if I wanted a simple function that asked if there is an offset from my normal timezone, it'd look something like this:

julia> is_offset(zdt::ZonedDateTime) = zdt.zone.offset.dst != Second(0)

julia> is_offset(ZonedDateTime(dt, tz"America/New_York"))
true

julia> is_offset(ZonedDateTime(dt, tz"Europe/Berlin"))
false

So right now, New York has an offset from its "standard" timezone (and we call that offset daylight savings), but Berlin does not (it transitioned back yesterday)

Does JavaScript's Date object automatically handle daylight savings?

Javascript Date objects use a time value that is an offset in milliseconds since 1970-01-01T00:00:00Z. It is always UTC.

If the Date constructor is given a single number argument, it is treated as a UTC time value, so represents the same instant in time regardless of system time zone settings.

When you use console.log(date), the built–in toString method is called which generates an implementation dependent string, generally using the current time zone setting of the host system to create a convenient, human readable string.

The current daylight saving rules in the system are used to determine the offset to use for "local" time, so if the date changes from a time when daylight saving applies to one when it doesn't, the time zone offset will be similarly adjusted (note that daylight saving offsets aren't always 1 hour). It does not matter what the current system offset is, the one used is based on the setting for the date and time that the time value represents.

Also, Date objects are very simple, they're just a time value. The time zone offset comes from system settings, it's not a property of the Date itself.

So, given:

My question is, if I were to run this same code again after the 30th
October when the clocks have gone back, would I still get the same
result of 23:30, or would it be 22:30 as if it were GMT?

the answer is "yes", it will still be 23:30 since on 13 September BST applies. It doesn't matter when the code is run, only what the system offset setting is for that date.

How to check if a time offset is in daylight savings time?

You cant derive this from the offset alone because the offset applies to the whole latitude and countries on that do not have uniform DST in effect. For instance in Australia, Daylight Saving Time is not observed in Queensland, the Northern Territory or Western Australia but everywhere else. You'd have to geolocate your users and then check with PHP's DateTimeZone API whether there is DST in that country.

Also see How to automatically detect user's timezone?

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