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How to check if the DST (Daylight Saving Time) is in effect and if it is what's the offset?

Posted by: admin November 29, 2017 Leave a comment

Questions:

This is a bit of my JS code for which this is needed:

var secDiff=Math.abs(Math.round((utc_date-this.premiere_date)/1000));
this.years=this.calculateUnit(secDiff,(86400*365));
this.days=this.calculateUnit(secDiff-(this.years*(86400*365)),86400);
this.hours=this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)),3600);
this.minutes=this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)-(this.hours*3600)),60);
this.seconds=this.calculateUnit((secDiff-(this.years*(86400*365))-(this.days*86400)-(this.hours*3600)-(this.minutes*60)),1);

I want to get the datetime in ago, but if the DST is in use then the dates are off by 1 hour. I don’t know how to check if the DST is in use or not.

How can i know when the daylight saving starts and ends?

Answers:

The code given by outdated link will tell you whether Daylight Savings Time is in effect. It uses the fact that getTimezoneOffset returns a different value during DST and standard time, and compares the difference between the two. (for example New York returns -5 normally and -4 during DST)

Note that I have no idea as to the intricacies of international time zones, and have only tested that it returns correct results for my time zone.. but the code seems solid.

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

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.dst = function() {
    return this.getTimezoneOffset() < this.stdTimezoneOffset();
}

Questions:
Answers:

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 timezone is DST (+1 hour)
  • if equal to January, southern hemisphere, then current timezone is DST (+1 hour)
Questions:
Answers:

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.

Questions:
Answers:

Based on Matt Johanson’s comment on the solution provided by Sheldon Griffin I created the following code:

    Date.prototype.stdTimezoneOffset = function() {
        var fy=this.getFullYear();
        if (!Date.prototype.stdTimezoneOffset.cache.hasOwnProperty(fy)) {

            var maxOffset = new Date(fy, 0, 1).getTimezoneOffset();
            var monthsTestOrder=[6,7,5,8,4,9,3,10,2,11,1];

            for(var mi=0;mi<12;mi++) {
                var offset=new Date(fy, monthsTestOrder[mi], 1).getTimezoneOffset();
                if (offset!=maxOffset) { 
                    maxOffset=Math.max(maxOffset,offset);
                    break;
                }
            }
            Date.prototype.stdTimezoneOffset.cache[fy]=maxOffset;
        }
        return Date.prototype.stdTimezoneOffset.cache[fy];
    };

    Date.prototype.stdTimezoneOffset.cache={};

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

It tries to get the best of all worlds taking into account all the comments and previously suggested answers and specifically it:

1) Caches the result for per year stdTimezoneOffset so that you don’t need to recalculate it when testing multiple dates in the same year.

2) It does not assume that DST (if it exists at all) is necessarily in July,
and will work even if it will at some point and some place be any month.
However Performance-wise it will work faster if indeed July (or near by months) are indeed DST.

3) Worse case it will compare the getTimezoneOffset of the first of each month. [and do that Once per tested year].

The assumption it does still makes is that the if there is DST period is larger then a single month.

If someone wants to remove that assumption he can change loop into something more like whats in the solutin provided by Aaron Cole – but I would still jump half a year ahead and break out of the loop when two different offsets are found]

Questions:
Answers:

Already solved, but just thought that I should add the way I do it (in case people are interested).

 function isDST(t) { //t is the date object to check, returns true if daylight saving time is in effect.
    var jan = new Date(t.getFullYear(),0,1);
    var jul = new Date(t.getFullYear(),6,1);
    return Math.min(jan.getTimezoneOffset(),jul.getTimezoneOffset()) == t.getTimezoneOffset();  
}

Works the same way as the excepted answer (janurary and july timezone offset), then returns the conditional.

Questions:
Answers:

Your’re close but a little off. You never need to calculate your own time as it is a result of your own clock. It can detect if you are using daylight saving time in your location but not for a remote location produced by the offset:

newDateWithOffset = new Date(utc + (3600000*(offset)));

This will still be wrong and off an hour if they are in DST. You need for a remote time account if they are currently inside their DST or not and adjust accordingly. try calculating this and change your clock to – lets say 2/1/2015 and reset the clock back an hour as if outside DST. Then calculate for an offset for a place that should still be 2 hours behind. It will show an hour ahead of the two hour window. You would still need to account for the hour and adjust. I did it for NY and Denver and always go the incorrect (hour ahead) in Denver.

Questions:
Answers:

I recently needed to create a date string with UTC and DST, and based on Sheldon’s answer I put this together:

Date.prototype.getTimezone = function(showDST) {
    var jan = new Date(this.getFullYear(), 0, 1);
    var jul = new Date(this.getFullYear(), 6, 1);

    var utcOffset = new Date().getTimezoneOffset() / 60 * -1;
    var dstOffset = (jan.getTimezoneOffset() - jul.getTimezoneOffset()) / 60;

    var utc = "UTC" + utcOffset.getSign() + (utcOffset * 100).preFixed(1000);
    var dst = "DST" + dstOffset.getSign() + (dstOffset * 100).preFixed(1000);

    if (showDST) {
        return utc + " (" + dst + ")";
    }

    return utc;
}
Number.prototype.preFixed = function (preCeiling) {
    var num = parseInt(this, 10);
    if (preCeiling && num < preCeiling) {
        num = Math.abs(num);
        var numLength		 = num.toString().length;
        var preCeilingLength = preCeiling.toString().length;
        var preOffset		 = preCeilingLength - numLength;
        for (var i = 0; i < preOffset; i++) {
            num = "0" + num;
        }
    }
    return num;
}
Number.prototype.getSign = function () {
    var num	 = parseInt(this, 10);
    var sign = "+";
    if (num < 0) {
        sign = "-";
    }
    return sign;
}

document.body.innerHTML += new Date().getTimezone() + "<br>";
document.body.innerHTML += new Date().getTimezone(true);
<p>Output for Turkey (UTC+0200) and currently in DST: &nbsp; UTC+0300 (DST+0100)</p>
<hr>

Questions:
Answers:

The moment.js library provides an .isDst() method on its time objects.

moment#isDST checks if the current moment is in daylight saving time.

moment([2011, 2, 12]).isDST(); // false, March 12 2011 is not DST
moment([2011, 2, 14]).isDST(); // true, March 14 2011 is DST

Questions:
Answers:

I’ve found that using the Moment.js library with some of the concepts described here (comparing Jan to June) works very well.

This simple function will return whether the timezone that the user is in observes Daylight Saving Time:

function HasDST() {
    return moment([2017, 1, 1]).isDST() != moment([2017, 6, 1]).isDST();
}

A simple way to check that this works (on Windows) is to change your timezone to a non DST zone, for example Arizona will return false, whereas EST or PST will return true.

enter image description here

Questions:
Answers:

The getTimezoneOffset() method in JavaScript, in a browser, returns the number of minutes offset from the 00:00 time zone. For example, America/New_York time zone in Daylight Savings (DST) returns the number 300. 300 minutes is 5 hours difference from zero. 300 minutes divided by 60 minutes is 5 hours. Every time zone is compared to the zero time zone, +00:00 / Etc/GMT / Greenwich time.

MDN Web Docs

The next thing that you must know, is that the offset has the opposite sign of the actual time zone.

Information about time zones is maintained by the Internet Assigned Numbers Authority (iana)

iana time zones

A nicely formatted table of Time Zones is supplied by joda.org

joda-time Time Zones

+00:00 or Etc/GMT is Greenwich time

All time zones are offset from +00:00 / “Etc/GMT” / Greenwich time

Daylight Savings Time is always an earlier time than the “regular” time in the summer. You set your clocks back in the fall season. (“Fall Back” slogan to remember what to do)

So, America/New_York time in Daylight Savings (winter) is one hour before the regular time. So, for example, what was normally 5 p.m. in the afternoon in New York city in the summer, is now 4 p.m. America/New_York time in Daylight Savings. The name “America/New_York” time is a “Long Format” time zone name. The east coast of the U.S typically calls their time zone Eastern Standard Time (EST)

If you want to compare today’s time zone offset to the time zone offset of some other date, you need to know that mathematical sign (+/- “Positive / Negative”) of the time zone offset is the opposite of the time zone.

Look at the time zone table at joda.org and find the time zone for “America/New_York” It will have a negative sign in front of the Standard Offset.

The earth rotates counter-clockwise on it’s axis. A person watch the sunrise in Greenwich sees the sunrise 5 hours before someone in New York City will see the sunrise. And someone on the West Coast of the U.S. will see the sunrise after someone on the East Coast of the U.S. sees the sunrise.

There’s a reason why you need to know all of this. So that you’ll be able to logically determine whether some JavaScript code is getting the DST status correctly or not, without needing to test every time zone at different times of the year.

Imagine that it’s November in New York City, and the clocks have been set back an hour. In the summer in New York City, the offset is 240 minutes or 4 hours.

You can test this by creating a date that is in July and then getting the offset.

var July_Date = new Date(2017, 6, 1);
var july_Timezone_OffSet = July_Date.getTimezoneOffset();

console.log('july_Timezone_OffSet: ' + july_Timezone_OffSet)

What will print to the browser’s developer tools console log?

Answer is: 240

So, now you can create a date in January and see what your browser returns for a time zone offset for the winter season.

var Jan_Date = new Date(2017, 0, 1);//Month is zero indexed - Jan is zero
var jan_Timezone_OffSet = Jan_Date.getTimezoneOffset();

console.log('jan_Timezone_OffSet: ' + jan_Timezone_OffSet)

Answer is: 300

Obviously 300 is bigger than 240. So, what does this mean? Should you write code that tests for the winter offset being bigger than the summer offset? Or the summer offset less than the winter offset? If there is a difference between the summer and winter time zone offsets, then you can assume that DST is being used for this time zone. But that doesn’t tell you if today is using DST for the browsers time zone. So, you’ll need to get the time zone offset for today.

var today = new Date();
var todaysTimeZone = today.getTimezoneOffset();

console.log('todaysTimeZone : ' + todaysTimeZone)

Answer is: ? – Depends on the time of year

If today’s time zone offset and the summer time zone offset is the same, AND the summer and winter time zone offsets are different, then by logical deduction, today must be NOT be in DST.

Can you omit comparing the summer and winter time zone offsets, (To know if DST is used for this time zone) and just compare today’s time zone offset to the summer TZ offset, and always get the correct answer?

today's TZ Offset !== Summer TZ Offset

Well, is today in the winter or summer? If you knew that then you could apply the following logic:

if ( it_is_winter && ( todays_TZ_Offset !== summer_TZ_Offset) {
  var are_We_In_DST = true;
}

But the problem is, that you don’t know if today’s date is in winter or summer. Every time zone can have it’s own rules for when DST starts and stops. You’d need to keep track of every time zone’s rules for every time zone in the world. So, if there is a better and easier way then you might as well do it the better and easier way.

What we are left with, is that you need to know if this time zone uses DST, and then compare today’s time zone offset with the summer time zone offset. That will always give you a reliable answer.

The final logic is:

if ( DST_Is_Used_In_This_Time_Zone && ( todays_TZ_Offset !== summer_TZ_Offset) {
  var are_We_In_DST = true;
}

Function to determine if the time zone in the browser uses DST:

function is_DST_Used_In_This_TimeZone() {
  var Jan_Date, jan_Timezone_OffSet, July_Date, july_Timezone_OffSet 
      offsetsNotEqual, thisYear, today;

  today = new Date();//Create a date object that is now
  thisYear = today.getFullYear();//Get the year as a number

  Jan_Date = new Date(thisYear, 0, 1);//Month is zero indexed - Jan is zero
  jan_Timezone_OffSet = Jan_Date.getTimezoneOffset();

  console.log('jan_Timezone_OffSet: ' + jan_Timezone_OffSet)

  July_Date = new Date(thisYear, 6, 1);
  july_Timezone_OffSet = July_Date.getTimezoneOffset();

  console.log('july_Timezone_OffSet: ' + july_Timezone_OffSet)

  offsetsNotEqual = july_Timezone_OffSet !== jan_Timezone_OffSet;//True if not equal

  console.log('offsetsNotEqual: ' + offsetsNotEqual);

  return offsetsNotEqual;//If the offsets are not equal for summer and
       //winter then the only possible reason is that DST is used for
       //this time zone
}