Javascript new Date decreases date by a day - javascript

I have a scenario where I am using AngularJS to read date. Interestingly it decreases my date value by one.
Why is this happening?
new Date("2016-01-06T00:00:00")
give me result as
Tue Jan 05 2016 16:00:00 GMT-0800 (Pacific Standard Time)

This is because when you use the new Date() in the JavaScript, it converts and prints the date in browsers timezone.
So if you print:
new Date("2016-01-06T00:00:00-0800")
You will get the actual output you want, because of the -0800 difference between your time zone (determined by the browser) and the UTC time.

The UTC time zone is used to interpret arguments in ISO 8601 format that do not contain time zone information (note that ECMAScript 2015 specifies that date time strings without a time zone are to be treated as local, not UTC).
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse
Since your date string appears to lack one, JS assumed it's UTC time. The result you see is the same moment in time, offset to your timezone. All you need to do is provide timezone data to the string you're parsing.

It is because the date is taking your browser's timezone in to account, which in your case is in PST. It does the same to me, in EST:
test = new Date("2016-01-06T00:00:00")
Tue Jan 05 2016 19:00:00 GMT-0500 (EST)
You can still obtain the time in UTC by using any of the .getUTC* functions, like so:
test.getUTCDate();
6

Related

.NET date, Moment.js, UTC and Timezone shifting

I've got a UTC date through an Ajax call, e.g. "/Date(1517216466000+0100)/",
which is when printing to the console: Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time).
What I need to do, is to let the user change the timezones: I can easily do it with e.g. moment(myDate).tz("Japan").
Then I need to save the date in it's UTC format, which I am not able to do.
I've been experimenting with moment.utc(), but for the input above, it returns 1 hour less.
How to deal with this situation? In summary:
1. Get a UTC time from a webservice
2. Let the user change the timezones
3. Save the modified date in UTC (without the timezone)
Working demo: https://stackblitz.com/edit/angular-kqrct7?file=app%2Fapp.component.html
EDIT for clarification:
Let's just have a look at the hours. What I get the date from the WCF, is 10 o'clock. The browser interprets it as 10 o'clock BUT in GMT+1 so when I convert it to UTC, it becomes 9 o'clock.
I want it to be 10 o'clock as UTC. Then, if I modify the timezone and e.g. the minutes of this date, I want to be able to get the UTC value of this date.
EDIT2: Made my question simplier for clarification
I've got a UTC date, which I get from a webservice like: "/Date(1517216466000+0100)/" which is: Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time) when printed to console.
I add a timezone to it with moment(this.inputDate).tz("Europe/Berlin").format(), but it stays 10:01:06, I guess because of my browsers GMT+1.
I want the ORIGINAL string to be used as a UTC date AND it should remain 10:01:06, not 09:01:06 as you can see above (2nd moment example), so with the timezone "Europe/Berlin" would be 11:01:6
In the .NET JSON formatted date "/Date(1517216466000+0100)/" the timezone offset can be ignored. It represents "2018-01-29T09:01:06.000Z", where the source system was at a timezone offset of +0100. So if you don't care about the source timezone, just ignore it.
It is also an identical moment in time to Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time), just with a different offset.
UTC is not a format, it's a time standard. If you want to use ISO 8601 format:
Extract the first numeric value
Convert to Number
Pass to the Date constructor
Call the toISOString method on the resulting Date
var s = '/Date(-1517216466000+0100)/';
console.log(new Date(+s.replace(/^[^\d-]+(-?\d+).*$/,'$1')).toISOString());
You can also parse and format it using moment.js, which according to the documentation can handle the .NET JSON format without needing to specify the format. So you can either do that or extract the time value and parse it with the "x" format token:
var s = '/Date(1517216466000+0100)/';
// Let moment.js guess the format
console.log(moment(s).utc());
// Extract time value and supply format
console.log(moment(s.replace(/^[^\d-]+(-?\d+).*$/,'$1'), 'x').utc());
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.20.1/moment.min.js"></script>
"/Date(1517216466000+0100)/" is a non-standard way to serialise a date/time. Take a look at ISO8601 which defines several standard ways to represent dates and times.
That being said let's take a look at what this gets evaluated as...
moment("/Date(1517216466000+0100)/").toDate()
gives Mon Jan 29 2018 09:01:06 GMT+0000 (GMT Standard Time) for me (in the UK)
taking just the timestamp value 1517216466000
new Date(1517216466000)
also gives Mon Jan 29 2018 09:01:06 GMT+0000 (GMT Standard Time)
this means that the +0100 is being ignored.
You're not actually modifying the time so why would you expect it to save back as anything other than Mon Jan 29 2018 09:01:06
UPDATE
But the "original" string represents Mon Jan 29 2018 09:01:06 UTC the +0100 is ignored and it's just coincidence that your offset to UTC is also +0100. An off.
Offset and timezone are 2 different things. A timezone encompasses offset to UTC as well as when/if daylight savings comes in to force. Just because it says +0100 doesn't necessarily mean (W. Europe Standard Time) as it could just as easily be (West Africa Time) which is also UTC+0100 but doesn't observe daylight savings at all.
The time you have "/Date(1517216466000+0100)/" doesn't convey enough information to say which timezone it is and JS/moment just uses the timestamp 1517216466000 and as such uses UTC. When you console.log() this the browser writes it to screen as local time Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time) but this is only a representation of the underlying datetime.
By telling moment to use a specific time zone, it's only changing how the date/time gets displayed and doesn't actually change the time it represents.
If you use a date picker to change the date/time then you'll have to serialise the value to send to the backend in an appropriate way that the .Net app you cannot change will understand and conveys what you intend.
Example
Get date from server as var serverTime = "/Date(1517216466000+0100)/"
convert this to a JS Date using moment var time = new moment(serverTime)
Let user specify TimeZone i.e. Japan Standard Time (UTC+9) time = time.tz("Japan")
time still represents Mon Jan 29 2018 09:01:06 UTC but when displayed on screen with time.format() gives "2018-01-29T18:01:06+09:00"
You stated "I want it to be 10 o'clock as UTC". Unfortunately the value you've is NOT 10 O'clock UTC and will never be 10 O'clock UTC because it isn't. It is 9 O'clock UTC
You could parse the value you get from the server yourself but the time from the server IS 9am UTC. If you change it to 10 am UTC then you would see that as Mon Jan 29 2018 11:01:06 GMT+0100 (W. Europe Standard Time) - 11 O'clock local time.
Thanks everyone for your detailed answers, they helped me a lot in understanding my problem! However, the right solution was the following:
10 o'clock was a UTC time in the database and it got interpreted as 9 o'clock UTC in Moment.js because C# handled it as a local time. So, before sending the date to the client, I had to indicate that its a UTC:
var utcToClient = DateTime.SpecifyKind(downtime.DownTimeStartUTC, DateTimeKind.Utc)
Then, in Moment I could create a UTC with:
var jsUtc = moment.utc(downtime.DownTimeStartUTC)
Changing the timezones was a breeze with:
jsUtc.tz(userSelectedTimezone)
And saving the date in the database, I used this in C#:
var utcFromClient = Record.DownTimeStartUTC.ToUniversalTime()

new Date() converts timezone

Why is new Date() converting the timezone? I'd like my date to be the same as the string I provide, so 00:30 and not 10:30.
>>> new Date("2015-04-11T00:30:00");
Sat Apr 11 2015 10:30:00 GMT+1000 (AEST)
You passed the date in ISO form into the constructor "2015-04-11T00:30:00".
That means your browser interprets that not as local time but as UTC. Date.toString however uses your local time. If you want to use UTC time call .toUTCString or better yet .toISOString.

Javascript new Date(dateString) handling

Would someone explain why formatting the same dateString differently gives a different date?
> new Date("04/08/1984")
<· Sun Apr 08 1984 00:00:00 GMT-0600 (Mountain Daylight Time)
> new Date("1984-04-08")
<· Sat Apr 07 1984 18:00:00 GMT-0600 (Mountain Daylight Time)
When you create a new Date object passing a dateString parameter to the constructor, it gets parsed using the Date.parse() method. Now, quoting from the MDN documentation (emphasis mine):
Differences in assumed time zone
Given a date string of "March 7, 2014" (or "03/07/2014"), parse() assumes a local time zone, but given an ISO format such as "2014-03-07" it will assume a time zone of UTC. Therefore Date objects produced using those strings will represent different moments in time unless the system is set with a local time zone of UTC.
Therefore, since that your are giving the second string in the ISO format, and your local time zone is UTC+6, you're getting a date which is six hour behind yours, because it gets calculated as UTC+0. In fact:
Apr 07 1984 18:00:00 = Apr 08 1984 00:00:00 - 06:00:00
Mystery solved!
Your problem is that you are adding 0 before the numbers in "1984-04-08". Try the following:
new Date("1984-4-8")
document.write(new Date("04/08/1984"));
document.write("<br>");
document.write(new Date("1984-4-8"));

Inconsistencies when creating new date objects

I am creating new date objects in javascript and seeing some inconsistencies depending on whether I use the dateString parameter vs the year/month/day integer parameters.
Here's an example:
var dt1 = new Date(1979,3,5);
var dt2 = new Date('1979-04-05');
jsFiddle with example
dt1 is assigned the value: Thu Apr 05 1979 00:00:00 GMT-0500 (Central Daylight Time)
dt2 is assigned the value: Wed Apr 04 1979 19:00:00 GMT-0500 (Central Daylight Time)
Can someone explain this behavior? The second example (dt2) happens to be the format that Chrome is returning a selected date from input[type=date] elements which is why I'm trying to figure this out.
It looks like the form '1979-04-05' is interpreted as a UTC date (and then that UTC date is converted to local time when displayed). The form new Date(1979,3,5); is interpreted as local time. You can use Date.UTC to force UTC time for the 3-argument form (see docs).
Date parsing (and timezone handling in particular) is generally not uniform across browsers, and it's better not to depend on it - use UTC whenever possible, or use a separate library like Date.js or moment.js.

Difference between these dates in Javascript? And why do not all browsers give same result?

var dt = new Date("2012-04-23T12:00:00");
var dtz = new Date("2012-04-23T12:00:00Z");
If the Z is present I get a different time.
When the Z is present is it converting the Date to the browser's local time and when not present assuming it is already in local time?
I get different results in FF than Chrome. Chrome always gives me the same time. FF treats them as different. How should I be dealing with UTC dates from the server?
"Z" is a military time zone corresponding to UT (aka UTC, aka GMT). So basically, 'nnn Z' means "how late is it in your time zone when it's 'nnn' in Greenwich". For example, I'm in CEST which is GMT+2 so this
new Date("2012-04-23T12:00:00Z")
returns for me:
Mon Apr 23 2012 14:00:00 GMT+0200 (CEST)
As to dates with a TZ specifier, they seem to be treated differently in Firefox (which assumes local TZ) and Chrome (which assumes UTC). For safety, I'd suggest always using an explicit TZ specifier.
var dt = new Date("2012-04-23T12:00:00");
var dtz = new Date("2012-04-23T12:00:00Z");
tried it with alert() and got these messages
alert(dt);
Mon Apr 23 2012 12:00:00 GMT+0500 (West Asia Standard Time)
alert(dtz);
Mon Apr 23 2012 17:00:00 GMT+0500 (West Asia Standard Time)
it means that if you create the date without "Z", it returns browsers's local time at GMT, mentioning your time zone is below or above GMT
and if you create it with "Z", it will show the local time at your time zone, referring to your time zone.
According to ISO 8601, IF no UTC relation information is given with a time representation, the time is assumed to be in local time.
If can verify the correct behavior on both Firefox, Safari and Internet Explorer:
The following should return: false
new Date("2014-05-09T22:12:18.893Z").valueOf() === new Date("2014-05-09T22:12:18.893").valueOf()
If you try the same thing on Chrome or Opera, it will incorrect indicate: true
The moral of the story is, if you have a string in the above format, add a Z at the end.

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