CODEWITHSUNDEEP
javascriptdatedatetimetimezonemomentjs
user8830344
user8830344

Reputation:

get time zone of historic date in javascript

A business requirement of a project is as follows:

The user needs to view historic dates based on his local time zone. For now, we store two dates in the database: UTC and the local time. The user needs to view the date in the (historic) time zone.

Example:

Database values: 2017-10-25T12:00:00 (UTC), 2017-10-25T09:00:00 (Date of registration).

From these values, we want the user to view the date in his time zone.

2017-10-25T12:00:00 (UTC) to CEST (time zone offset +0200) => 2017-10-25T14:00:00.

However, when the user views the same date on a date where summer time does not apply, the user will see 2017-10-25T13:00:00 because CET is +0100.

I'm looking for a way in which it is possible to get the offset from UTC for the user's locale on the UTC date.

In short: get the UTC offset for a historic date in the user's local time zone.

EDIT: libraries like Moment.js and Moment_timezone.js are accepted!

Upvotes: 1

Views: 655

Answers (2)

user8830344
user8830344

Reputation:

Thank you all for your comments. I think I got it by doing like so:

var oUTC = moment(new Date(2015, 7, 25, 12, 0, 0, 0));
var oRecordedDate = moment(new Date(2015, 7, 25, 13, 0, 0, 0));

//step 1: calculate historic offset with DB values
var iDBOffset = oUTC.diff(oRecordedDate);

//step 2: calculate historic offset from UTC in user's locale
var oHistoricDate = oUTC.clone().tz(moment.tz.guess());
var iHistoricOffset = Math.abs(oHistoricDate.utcOffset() * 60 * 1000);

//step 3: calculate total offset
var iTotalOffset = iDBOffset - iHistoricOffset;

//step 4: resulting date object
var oDate = new Date(oRecordedDate.valueOf() - iTotalOffset);
console.log(oDate.toString());

Upvotes: 0

Matt Johnson-Pint
Matt Johnson-Pint

Reputation: 241525

I'm looking for a way in which it is possible to get the offset from UTC for the user's locale on the UTC date.

var dt = new Date("2017-10-25T12:00:00Z");
var offset = dt.getTimezoneOffset();

(note the Z at the end)

Though really, one wonders why you need the offset at all. The local time is already represented by the Date object in dt.

If you want to do this with Moment, then you can do:

var m = moment("2017-10-25T12:00:00Z");

Or if you don't want to add the Z then create the Moment object like so:

var m = moment.utc("2017-10-25T12:00:00").local();

You can format the moment from there, or call m.utcOffset() if you like.

You don't need Moment-timezone, unless you intend to reflect some time zone other than the user's local time zone.

Upvotes: 1

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