Alternative to casting UTC Date in Javascript? - javascript

I wish to create a new Date in JS, but have it be cast as UTC time. For example, suppose castAsUTC() produces the following desired effect:
var x = new Date('2019-01-01T00:00:00') // In local time (PST)
castAsUTC(x).toISOString(); // => '2019-01-01T00:00:00Z'
// x.toISOString() gives us '2019-01-01T08:00:00Z', which is undesired
Currently, my function looks like this:
function castAsUTC(date) {
return new Date(x.toLocaleString() + '+00:00');
}
Is there a cleaner/nicer way of producing the same effect? Thanks in advance!
EDIT: To be more specific, I'm interested in transforming the date's timezone, without changing its actual value with as little arithmetic as possible. So calling .toISOString() will produce the same date as it is in local time.
I am currently using the moment-timezone library, but I can't seem to get the desired effect using that, either. I would definitely accept an answer that uses Moment.js

You can switch a Moment instance to UTC using the utc function. Then just use format to get whatever the specific output you want from it.

If indeed the string you have is like the one shown, then the easiest thing to do would be to append a Z to indicate UTC.
var input = '2019-01-01T00:00:00';
var date = new Date(input + 'Z');
var output = date.toISOString();
Or, if you would like to use Moment.js, then do this:
var input = '2019-01-01T00:00:00';
var m = moment.utc(input);
var output = m.format();
You do not need moment-timezone for this.

tl;dr;
You formatted the date wrong. Add the letter "Z" to the end of your date string and it will be treated as UTC.
var x = new Date('2019-01-01T00:00:00Z') // Jan 1, 2019 12 AM UTC
These formatting issues are easier to manage with a library like momentjs (utc and format functions) as described in other answers. If you want to use vanilla javascript, you'll need to subtract out the timezone offset before calling toISOString (see warnings in the longer answer below).
Details
Date in javascript deals with timezones in a somewhat counter intuitive way. Internally, the date is stored as the number of milliseconds since the Unix epoch (Jan 1, 1970). That's the number you get when you call getTime() and it's the number that's used for math and comparisons.
However - when you use the standard string formatting functions (toString, toTimeString, toDateString, etc) javascript automatically applies the timezone offset for the local computers timezone before formatting. In a browser, that means it will apply the offset for the end users computer, not the server. The toISOString and toUTCString functions will not apply the offset - they print the actual UTC value stored in the Date. This will probably still look "wrong" to you because it won't match the value you see in the console or when calling toString.
Here's where things really get interesting. You can create Date's in javascript by specifying the number of milliseconds since the Unix epoch using new Date(milliseconds) or by using a parser with either new Date(dateString). With the milliseconds method, there's no timezone to worry about - it's defined as UTC. The question is, with the parse method, how does javascript determine which timezone you intended? Before ES5 (released 2009) the answer was different depending on the browser! Post ES5, the answer depends on how you format the string! If you use a simplified version of ISO 8601 (with only the date, no time), javascript considers the date to be UTC. Otherwise, if you specify the time in ISO 8601 format, or you use a "human readable" format, it considers the date to be local timezone. Check out MDN for more.
Some examples. I've indicated for each if javascript treats it as a UTC or a local date. In UTC, the value would be Jan 1, 1970 at midnight. In local it depends on the timezone. For OP in pacfic time (UTC-8), the UTC value would be Jan 1, 1970 at 8 AM.
new Date(0) // UTC (milliseconds is always UTC)
new Date("1/1/1970"); // Local - (human readable string)
new Date("1970-1-1"); // Local (invalid ISO 8601 - missing leading zeros on the month and day)
new Date("1970-01-01"); // UTC (valid simplified ISO 8601)
new Date("1970-01-01T00:00"); // Local (valid ISO 8601 with time and no timezone)
new Date("1970-01-01T00:00Z"); // UTC (valid ISO 8601 with UTC specified)
You cannot change this behavior - but you can be pedantic about the formats you use to parse dates. In your case, the problem was you provided an ISO-8601 string with the time component but no timezone. Adding the letter "Z" to the end of your string, or removing the time would both work for you.
Or, always use a library like momentjs to avoid these complexities.
Vanilla JS Workaround
As discussed, the real issue here is knowing whether a date will be treated as local or UTC. You can't "cast" from local to UTC because all Date's are UTC already - it's just formatting. However, if you're sure a date was parsed as local and it should really be UTC, you can work around it by manually adjusting the timezone offset. This is referred to as "epoch shifting" (thanks #MattJohnson for the term!) and it's dangerous. You actually create a brand new Date that refers to a different point in time! If you use it in other parts of your code, you can end up with incorrect values!
Here's a sample epoch shift method (renamed from castAsUtc for clarity). First get the timezone offset from the object, then subtract it and create a new date with the new value. If you combine this with toISOString you'll get a date formatted as you wanted.
function epochShiftToUtc(date) {
var timezoneOffsetMinutes = date.getTimezoneOffset();
var timezoneOffsetMill = timezoneOffsetMinutes * 1000 * 60;
var buffer = new Date(date.getTime() - timezoneOffsetMill);
return buffer;
}
epochShiftToUtc(date).toUTCString();

Related

Legit javascript UTC Date

I've been looking for a way to keep my dates in UTC in my JS application; however, in every case the Date's getTimezoneOffset() does not return a 0 which, I would imagine, should be the case -- and which would be seemingly important in casting dates between UTC and the local TZ.
See below examples of what I've tried, thanks in advance for any insight!
var TheDate = new Date( Date.UTC(2012, 10, 5) );
console.log(TheDate.getTimezoneOffset()) // => 300 (for me)
console.log(moment().utc().toDate().getTimezoneOffset()) // => 240 (for me)
ECAMScript Dates are inherently UTC and are just an offset from the ECMAScript epoch (a time value in milliseconds from 1970-01-01T00:00:00Z), nothing more. They have no associated timezone.
The host timezone offset is used:
When creating a Date to determine the equivalent UTC time and calculate the time value
In various methods that use local dates and times, such as getHours (vs getUTCHours)
The date itself does not know anything about timezones.
The value returned by getTimezoneOffset is based on the host system settings, the only relationship it has to the Date it's called on is that the offset is calculated for that Date. The method might be called getHostSystemTimezoneOffset, because that's what it returns.
In your code:
new Date(Date.UTC(2012, 10, 5))
creates a Date for 2012-11-05T00:00:00Z. Calling getTimezoneOffset on that date returns the host system offset for the equivalent local date and time, not the offset that was used in creating the Date.
There is no way to associate a timezone with a date without using a library (either one you write or one of the many existing libraries).
In your second example:
moment().utc()
just sets a flag to tell moment.js methods to use UTC for everything. Then using:
....toDate().getTimezoneOffset()
returns a Date object, then gets the host system timezone offset for that date as if you'd done:
new Date().getTimezoneOffset()
If you want to only use UTC, then use UTC methods for everything and ignore timezones completely (which I think is what you want to do).

How can I create a Date from string, ignoring any timezone offsets?

The server uses +03:00 timezone. It offers me a date in this format: "2017-04-12T00:00:00+03:00"
I then create a new Date from this string:
options.startDate = new Date("2017-04-12T00:00:00+03:00")
But because on the client there is a different timezone, the result is actually:
Tue Apr 11 2017 23:00:00 GMT+0200 (Central Europe Daylight Time)
This brings me back one day and it's a big deal for me. Is there an elegant way to avoid this and create the same Date and Time in javascript, ignoring the timezone offset?
The date you have in options.startDate is the correct one. What you want is to display it as if you were from the same timezone as the server.
If you now server's timezone in the client script then I would considere using a library like moment.js. It would allow you to format date in the timezone you want (GMT for instance, or the one of the server).
Using both moment.js and its plugin timezone code could be :
moment("2017-04-12T00:00:00+03:00").tz("America/Los_Angeles").format();
You should never use the Date constructor or Date.parse to parse strings due to browsers differences. Even if you remove the timezone from the string and parse the remainder, e.g.
console.log( new Date('2017-04-12T00:00:00+03:00'.substr(0,19)).toString() );
you'll get different results in different browsers (e.g. Firefox and Safari).
If you don't want to use a library, use a simple function (see below). However, if you remove the timezone, the string will represent a different moment in time in each timezone with a different offset.
function parseISOIgnoreTimezone(s) {
var b = s.split(/\D/);
return new Date(b[0], b[1]-1, b[2], b[3], b[4], b[5]);
}
console.log(parseISOIgnoreTimezone('2017-04-12T00:00:00+03:00').toString());
I really recommend #VictorDrouin his answer.
But if for some reason you don't want moment.js or fiddle around with it you can use this 'hack'
new Date("2017-04-12T00:00:00+03:00".match(/\d{4}\-\d{2}\-\d{2}T\d{2}:\d{2}:\d{2}/).pop());
What it does it matches the date against given regex date format, and then supplies it to the date parser which makes it a date.
Be careful when supplying it back to the database that you supply it back without timezone offset.
var stringdate = "2017-04-12T00:00:00+03:00";
function getDate(str_date) {
var matched = str_date.match(/\d{4}\-\d{2}\-\d{2}T\d{2}:\d{2}:\d{2}/).pop();
return new Date(matched);
}
console.log(getDate(stringdate));

Strip Time Zone from a string using JavaScripty, Moment.js, etc?

Suppose I have a string like "01/22/2014 2:07:00 PM -08:00".
I want to
a) Format it in ISO 8601 with time offsets from UTC http://goo.gl/JTfAZq, so it becomes "2014-01-22T14:07:00-08:00"
b) Strip out time offset part so it becomes "01/22/2014 2:07:00 PM" [and then format it in ISO 8601 so it becomes "2014-01-22T14:07:00"]
Sure I can use JavaScript string functions (and regular expressions), but it seems to be a better approach to use JavaScript Date() object facilities or Moment.js. Neither work, however. Both automatically convert dates to a current system timezone (-05:00 for me), so 2:07 PM becomes 5:07 PM. I found two ways of doing that "strip out time offset then format" task, but both looks ugly and brittle:
var mydateTime = "01/22/2014 2:07:00 PM -08:00";
// strip out time offset part using substring() so that Moment.js
// would think time is specified in a current zone
var myNewDateTime1 = moment(mydateTime.substring(0, mydateTime.length - 7)).format("YYYY-MM-DDTHH:mm:ss")
// or probably even worse trick - strip out time offset part using format
var myNewDateTime2 = moment(mydateTime, "MM/DD/YYYY h:mm:ss A").format("YYYY-MM-DDTHH:mm:ss")
I understand that JavaScript Date() object is not designed to preserve a time zone, but doesn't more elegant and stable solution exist for a) and b) ?
I think you are looking for moment.ParseZone. It parses the moment AND preserves the time zone offset that was in the string, instead of converting it to the browser's local time zone.
Also, your myDateTime variable doesn't match what you were asking about. If you do indeed already have a full ISO8601 extended with time zone offfset, then it is like this:
var m = moment.parseZone("2014-01-22T14:07:00-08:00");
Or if it's like you originally, showed, then like this:
var m = moment("01/22/2014 2:07:00 PM -08:00",
"MM/DD/YYYY h:mm:ss A Z").parseZone();
From there, you can format it however you like:
var s = m.format("YYYY-MM-DDTHH:mm:ss");

How do you check whether a date is UTC in Javascript?

I've read this question:
How do you convert a JavaScript date to UTC?
and based on this I implemented this conversion in a dateTools module as follows:
[Update]
var dt, utcTime;
dt = new Date();
utcTime = new Date(Date.UTC(dt.getFullYear(),
dt.getMonth(),
dt.getDate(),
dt.getHours(),
dt.getMinutes(),
dt.getSeconds(),
dt.getMilliseconds()));
Now I'd like to write unit tests. My idea was to check whether the result is actually in UTC, but I don't know how.
All the toString, toUTCString and similar methods seem to be identical for the input (non UTC) and output (UTC) date.
Only the result of the getTime method differs.
Is there a way to check wheter a date is UTC in javascript? If not, is there a better idea to unit test this?
To give more context:
Only converting the it to a UTC string is not that helpful, because in the next step the date is sent to an Asp.net service and therefore converted to a string like:
'/Date([time])/'
with this code
var aspDate = '/Date(' + date.getTime() + ')/';
var aspDate = '/Date(' + date.getTime() + ')/';
This outputs the internal UNIX epoch value (UTC), which represents a timestamp. You can use the various toString methods to get a more verbose string representation of that timestamp:
.toString() uses the users timezone, result is something like "Fri Jan 25 2013 15:20:14 GMT+0100" (for me, at least, you might live in a different timezone)
.toUTCString() uses UTC, and the result will look like "Fri, 25 Jan 2013 14:20:15 GMT"
.toISOString() uses UTC, and formats the datestring according to ISO: "2013-01-25T14:20:20.061Z"
So how do we construct the time value that we want?
new Date() or Date.now() result in the current datetime. No matter what the user's timezone is, the timestamp is just the current moment.
new Date(year, month, …) uses the users timezone for constructing a timestamp from the single values. If you expect this to be the same across your user community, you are screwed. Even when not using time values but only dates it can lead to odd off-by-one errors.
You can use the setYear, setMonth, … and getYear, getMonth … methods to set/get single values on existing dates according to the users timezone. This is appropriate for input from/output to the user.
getTimezoneOffset() allows you to query the timezone that will be used for all these
new Date(timestring) and Date.parse cannot be trusted. If you feed them a string without explicit timezone denotation, the UA can act random. And if you want to feed a string with a proper format, you will be able to find a browser that does not accept it (old IEs, especially).
Date.UTC(year, month, …) allows you to construct a timestamp from values in the UTC timezone. This comes in handy for input/output of UTC strings.
Every get/set method has a UTC equivalent which you can also use for these things.
You can see now that your approach to get the user-timezone values and use them as if they were in UTC must be flawed. It means either dt or utcTime has the wrong value, although using the wrong output method may let it appear correct.
getTimezoneOffset
Syntax: object.getTimezoneOffset( ) This method
returns the difference in minutes between local time and Greenwich
Mean Time. This value is not a constant, as you might think, because
of the practice of using Daylight Saving Time.
i.e.
var myDate = new Date;
var myUTCDate = new Date(myDate - myDate.getTimezoneOffset() * 60000);
alert(myUTCDate);
note: 60000 is the number of milliseconds in a minute;

json dates and timezones

I have the following code:
$(function () {
var thedate = "/Date(1198908717056)/";
var thedate2 = ProcessDate(thedate)
alert(thedate2);
});
function ProcessDate(DateString) {
var TheDate = eval(DateString.replace(/\/Date\((\d+)\)\//gi, "new Date($1)"));
return TheDate;
}
When it runs, it returns an alert with December 29 and the time is showing as Eastern Time. When I change the timezone on my computer, it's still showing the date in the Eastern timezone.
My question is this: does the string "/Date(1198908717056)/" contain the timezone information or is the timezone displayed in the alert the result of the browser determining my timezone?
Thanks.
JSON doesn't have dates at all (it's one of JSON's flaws). Those strings are just strings.
Some frameworks, like ASP.Net, use that syntax to indicate dates. What timezone they're in will be dictated by the framework. I believe the dates are in UTC and so you can just use the new Date(Number) constructor to create them (more in this other answer). That creates the date by directly setting its internal "milliseconds since The Epoch UTC" value, more in section 15.9 of the specification. Mind you, that only works if, in fact, whatever it is creating these pseudo-date strings is using UTC.
Update: Looking at your code, although it works, this line:
var TheDate = eval(DateString.replace(/\/Date\((\d+)\)\//gi, "new Date($1)"));
...is an abuse of eval. eval should be avoided whenever possible. Instead, if you want to keep it as a one-liner:
var TheDate = new Date(Number(DateString.replace(/\/Date\((\d+)\)\//gi, "$1")));
...or somewhat more readably:
var Match = /\/Date\((\d+)\)\//gi.exec(DateString);
var TheDate;
if (Match) {
TheDate = new Date(Number(Match[1]));
}
In all of those cases, the Date will be initialized with the UTC time value embedded in the date string. But then when you ask JavaScript to format the date (for instance, via toString), it will use your local timezone to do that. You haven't shown how you're outputting the date, so I can't tell you why the timezone seems not to change if you change your timezone (perhaps the browser didn't pick up the change?). When I do it, if I output the date as a string, it shows it in British Summer Time (which is my current timezone) if I use toString, or UTC if I use toUTCString. Here's a live example using both your original date, and a date (today's date, as I write this) that's in daylight savings time so even in the UK you can see the difference between UTC and local time.
Off-topic: In JavaScript, the overwhelming convention is to use camelCased names starting with a lower-case letter for both local variables and function names. So, theDate rather than TheDate. Initial caps are reserved for constructor functions (like Date). You're free to ignore the convention, of course, but it will tend to make it difficult for others to read your code.
The timezone is taken from your current system setting. Have a look at the Date class.
The given value is in milliseconds and does not contain a timezone. The constructor of Date() expects the milliseconds to be given in UTC. If you have values with a known timezone, you should use the dateString constructor version.
However, as far as I know, there is no way convert between timezones in JavaScript, except for UTC and the local system timezone.

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