How to ignore user's time zone and force Date() use specific time zone

In an JS app, I receive timestamp (eq. 1270544790922) from server (Ajax).

Basing on that timestamp I create Date object using:

var _date = new Date();
_date.setTime(1270544790922);

开发者_如何转开发Now, _date decoded timestamp in current user locale time zone. I don't want that.

I would like _date to convert this timestamp to current time in city of Helsinki in Europe (disregarding current time zone of the user).

How can I do that?

A Date object's underlying value is actually in UTC. To prove this, notice that if you type new Date(0) you'll see something like: Wed Dec 31 1969 16:00:00 GMT-0800 (PST). 0 is treated as 0 in GMT, but .toString() method shows the local time.

Big note, UTC stands for Universal time code. The current time right now in 2 different places is the same UTC, but the output can be formatted differently.

What we need here is some formatting

var _date = new Date(1270544790922); 
// outputs > "Tue Apr 06 2010 02:06:30 GMT-0700 (PDT)", for me
_date.toLocaleString('fi-FI', { timeZone: 'Europe/Helsinki' });
// outputs > "6.4.2010 klo 12.06.30"
_date.toLocaleString('en-US', { timeZone: 'Europe/Helsinki' });
// outputs > "4/6/2010, 12:06:30 PM"

This works but.... you can't really use any of the other date methods for your purposes since they describe the user's timezone. What you want is a date object that's related to the Helsinki timezone. Your options at this point are to use some 3rd party library (I recommend this), or hack-up the date object so you can use most of it's methods.

Option 1 - a 3rd party like moment-timezone

moment(1270544790922).tz('Europe/Helsinki').format('YYYY-MM-DD HH:mm:ss')
// outputs > 2010-04-06 12:06:30
moment(1270544790922).tz('Europe/Helsinki').hour()
// outputs > 12

This looks a lot more elegant than what we're about to do next.

Option 2 - Hack up the date object

var currentHelsinkiHoursOffset = 2; // sometimes it is 3
var date = new Date(1270544790922);
var helsenkiOffset = currentHelsinkiHoursOffset*60*60000;
var userOffset = _date.getTimezoneOffset()*60000; // [min*60000 = ms]
var helsenkiTime = new Date(date.getTime()+ helsenkiOffset + userOffset);
// Outputs > Tue Apr 06 2010 12:06:30 GMT-0700 (PDT)

It still thinks it's GMT-0700 (PDT), but if you don't stare too hard you may be able to mistake that for a date object that's useful for your purposes.

I conveniently skipped a part. You need to be able to define currentHelsinkiOffset. If you can use date.getTimezoneOffset() on the server side, or just use some if statements to describe when the time zone changes will occur, that should solve your problem.

Conclusion - I think especially for this purpose you should use a date library like moment-timezone.

To account for milliseconds and the user's time zone, use the following:

var _userOffset = _date.getTimezoneOffset()*60*1000; // user's offset time
var _centralOffset = 6*60*60*1000; // 6 for central time - use whatever you need
_date = new Date(_date.getTime() - _userOffset + _centralOffset); // redefine variable

Just another approach

function parseTimestamp(timestampStr) {
  return new Date(new Date(timestampStr).getTime() + (new Date(timestampStr).getTimezoneOffset() * 60 * 1000));
};

//Sun Jan 01 2017 12:00:00
var timestamp = 1483272000000;
date = parseTimestamp(timestamp);
document.write(date);

Cheers!

I have a suspicion, that the Answer doesn't give the correct result. In the question the asker wants to convert timestamp from server to current time in Hellsinki disregarding current time zone of the user.

It's the fact that the user's timezone can be what ever so we cannot trust to it.

If eg. timestamp is 1270544790922 and we have a function:

var _date = new Date();
_date.setTime(1270544790922);
var _helsenkiOffset = 2*60*60;//maybe 3
var _userOffset = _date.getTimezoneOffset()*60*60; 
var _helsenkiTime = new Date(_date.getTime()+_helsenkiOffset+_userOffset);

When a New Yorker visits the page, alert(_helsenkiTime) prints:

Tue Apr 06 2010 05:21:02 GMT-0400 (EDT)

And when a Finlander visits the page, alert(_helsenkiTime) prints:

Tue Apr 06 2010 11:55:50 GMT+0300 (EEST)

So the function is correct only if the page visitor has the target timezone (Europe/Helsinki) in his computer, but fails in nearly every other part of the world. And because the server timestamp is usually UNIX timestamp, which is by definition in UTC, the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT), we cannot determine DST or non-DST from timestamp.

So the solution is to DISREGARD the current time zone of the user and implement some way to calculate UTC offset whether the date is in DST or not. Javascript has not native method to determine DST transition history of other timezone than the current timezone of user. We can achieve this most simply using server side script, because we have easy access to server's timezone database with the whole transition history of all timezones.

But if you have no access to the server's (or any other server's) timezone database AND the timestamp is in UTC, you can get the similar functionality by hard coding the DST rules in Javascript.

To cover dates in years 1998 - 2099 in Europe/Helsinki you can use the following function (jsfiddled):

function timestampToHellsinki(server_timestamp) {
    function pad(num) {
        num = num.toString();
        if (num.length == 1) return "0" + num;
        return num;
    }

    var _date = new Date();
    _date.setTime(server_timestamp);

    var _year = _date.getUTCFullYear();

    // Return false, if DST rules have been different than nowadays:
    if (_year<=1998 && _year>2099) return false;

    // Calculate DST start day, it is the last sunday of March
    var start_day = (31 - ((((5 * _year) / 4) + 4) % 7));
    var SUMMER_start = new Date(Date.UTC(_year, 2, start_day, 1, 0, 0));

    // Calculate DST end day, it is the last sunday of October
    var end_day = (31 - ((((5 * _year) / 4) + 1) % 7))
    var SUMMER_end = new Date(Date.UTC(_year, 9, end_day, 1, 0, 0));

    // Check if the time is between SUMMER_start and SUMMER_end
    // If the time is in summer, the offset is 2 hours
    // else offset is 3 hours
    var hellsinkiOffset = 2 * 60 * 60 * 1000;
    if (_date > SUMMER_start && _date < SUMMER_end) hellsinkiOffset = 
    3 * 60 * 60 * 1000;

    // Add server timestamp to midnight January 1, 1970
    // Add Hellsinki offset to that
    _date.setTime(server_timestamp + hellsinkiOffset);
    var hellsinkiTime = pad(_date.getUTCDate()) + "." + 
    pad(_date.getUTCMonth()) + "." + _date.getUTCFullYear() + 
    " " + pad(_date.getUTCHours()) + ":" +
    pad(_date.getUTCMinutes()) + ":" + pad(_date.getUTCSeconds());

    return hellsinkiTime;
}

Examples of usage:

var server_timestamp = 1270544790922;
document.getElementById("time").innerHTML = "The timestamp " + 
server_timestamp + " is in Hellsinki " + 
timestampToHellsinki(server_timestamp);

server_timestamp = 1349841923 * 1000;
document.getElementById("time").innerHTML += "<br><br>The timestamp " + 
server_timestamp + " is in Hellsinki " + timestampToHellsinki(server_timestamp);

var now = new Date();
server_timestamp = now.getTime();
document.getElementById("time").innerHTML += "<br><br>The timestamp is now " +
server_timestamp + " and the current local time in Hellsinki is " +
timestampToHellsinki(server_timestamp);​

And this print the following regardless of user timezone:

The timestamp 1270544790922 is in Hellsinki 06.03.2010 12:06:30

The timestamp 1349841923000 is in Hellsinki 10.09.2012 07:05:23

The timestamp is now 1349853751034 and the current local time in Hellsinki is 10.09.2012 10:22:31

Of course if you can return timestamp in a form that the offset (DST or non-DST one) is already added to timestamp on server, you don't have to calculate it clientside and you can simplify the function a lot. BUT remember to NOT use timezoneOffset(), because then you have to deal with user timezone and this is not the wanted behaviour.

Presuming you get the timestamp in Helsinki time, I would create a date object set to midnight January 1 1970 UTC (for disregarding the local timezone settings of the browser). Then just add the needed number of milliseconds to it.

var _date	= new Date( Date.UTC(1970, 0, 1, 0, 0, 0, 0) );
_date.setUTCMilliseconds(1270544790922);

alert(_date); //date shown shifted corresponding to local time settings
alert(_date.getUTCFullYear());    //the UTC year value
alert(_date.getUTCMonth());       //the UTC month value
alert(_date.getUTCDate());        //the UTC day of month value
alert(_date.getUTCHours());       //the UTC hour value
alert(_date.getUTCMinutes());     //the UTC minutes value

Watch out later, to always ask UTC values from the date object. This way users will see the same date values regardless of local settings. Otherwise date values will be shifted corresponding to local time settings.

My solutions is to determine timezone adjustment the browser applies, and reverse it:

var timestamp = 1600913205;  //retrieved from unix, that is why it is in seconds

//uncomment below line if you want to apply Pacific timezone
//timestamp += -25200;

//determine the timezone offset the browser applies to Date()
var offset = (new Date()).getTimezoneOffset() * 60;   

//re-initialize the Date function to reverse the timezone adjustment
var date = new Date((timestamp + offset) * 1000);

//here continue using date functions.

This point the date will be timezone free and always UTC, You can apply your own offset to timestamp to produce any timezone.

Use this and always use UTC functions afterwards e.g. mydate.getUTCHours();

   function getDateUTC(str) {
        function getUTCDate(myDateStr){
            if(myDateStr.length <= 10){
                //const date = new Date(myDateStr); //is already assuming UTC, smart - but for browser compatibility we will add time string none the less
                const date = new Date(myDateStr.trim() + 'T00:00:00Z');
                return date;
            }else{
                throw "only date strings, not date time";
            }
        }

        function getUTCDatetime(myDateStr){
            if(myDateStr.length <= 10){
                throw "only date TIME strings, not date only";
            }else{
                return new Date(myDateStr.trim() +'Z'); //this assumes no time zone is part of the date string. Z indicates UTC time zone
            }
        }  
        
        let rv = '';
        
        if(str && str.length){
            if(str.length <= 10){
                rv = getUTCDate(str);
            }else if(str.length > 10){
                rv = getUTCDatetime(str);
            } 
        }else{
            rv = '';
        }
        return rv;
    }

console.info(getDateUTC('2020-02-02').toUTCString());

var mydateee2 = getDateUTC('2020-02-02 02:02:02');
console.info(mydateee2.toUTCString());

// you are free to use all UTC functions on date e.g.
console.info(mydateee2.getUTCHours())
console.info('all is good now if you use UTC functions')

You could use setUTCMilliseconds()

var _date = new Date();
_date.setUTCMilliseconds(1270544790922);