CODEWITHSUNDEEP
javascripttimezonetimezone-offset
Edgar Barber
Edgar Barber

Reputation: 385

new Date() returns relative time and not UTC time

When I go to my browser development tools and write new Date() in the console, it gives me the following

Mon Dec 18 2017 17:11:29 GMT+0200

I thought it suppose to return the UTC time.

The issue is that I have a server on AWS which writes UTC time to a DB. it writes it as a string and on the client side I do the following

const updatedMilliAgo = new Date() - new Date(timeStrFromDb);

For some reason the diff is two hours even due I check it right after the write in the server.

What am I doing wrong here?

Upvotes: 8

Views: 18468

Answers (3)

Migue Cobas
Migue Cobas

Reputation: 1

I am late, but I didn't find the answer and I had to fight for hours vs the code... finally my solution was as simple as that:

        TimeZone timeZone = TimeZone.getTimeZone(ZoneId.of("UTC"));
        timeZone.setDefault(timeZone);

When you set the default timezone, all dates you create will be in that ZoneId that you specify.

I hope to help to someone.

Upvotes: -1

Matt Johnson-Pint
Matt Johnson-Pint

Reputation: 241959

When you use new Date(), you are constructing a Date object using the current value of the system clock where the code is executing. The Date object internally stores only a number of milliseconds since 1970-01-01T00:00:00Z (without consideration of leap seconds). In other words, Date objects are always representing the UTC time.

However - there are many functions and parameters on the Date object that work in local time. For example, when you call .toString() on a Date object, the computer's local time zone is applied to the internal UTC-based value, in order to generate a string that reflects local time.

In the case of console.log - a standard object like Date cannot be directly logged. Instead, most implementations will log a string value. How that value is created is entirely implementation specific, and not defined by the ECMAScript specification. Many implementations will return the same local-time based value that .toString() returns. Some (FireFox, for example) will return the same UTC based value that .toISOString() returns. It would be reasonable for an implementation to return the actual number of milliseconds stored (.valueOf()), or some other representation. If you need consistency, don't just log the Date object. Instead, log the output of one of its functions that returns a string or a number.

You also asked about subtracting two date objects. That will implicitly call .valueOf() on each object, subtracting their UTC-based internal values and giving you the number of milliseconds between them. The most likely problem you are encountering is with how you construct the second Date object. You didn't give an example of what timeStrFromDb consists of, but understand that how that string is formatted directly relates to how the Date object is constructed. If you aren't using a standardized format, or you aren't clear on whether the value is based on UTC or a specific offset from UTC, your string may be parsed differently than you expect.

Upvotes: 10

GibboK
GibboK

Reputation: 74008

Try to use 

var d = new Date();
var n = d.toUTCString();
console.log(n)

Upvotes: 0

Related Questions