MomentJS getting JavaScript Date in UTC

Question

I am not able to get the JavaScript Date string for MongoDB record via the following. It keeps using my local time.

var utc = moment.utc().valueOf(); console.log(moment.utc(utc).toDate()); 

Output:

Tue Nov 11 2014 14:42:51 GMT-0500 (EST) 

I need it to be in UTC, so I can stick this timestamp in Mongo so type would be Date.

How can I do that?

Answer 1

A timestamp is a point in time. Typically this can be represented by a number of milliseconds past an epoc (the Unix Epoc of Jan 1 1970 12AM UTC). The format of that point in time depends on the time zone. While it is the same point in time, the "hours value" is not the same among time zones and one must take into account the offset from the UTC.

Here's some code to illustrate. A point is time is captured in three different ways.

var moment = require( 'moment' );  var localDate = new Date(); var localMoment = moment(); var utcMoment = moment.utc(); var utcDate = new Date( utcMoment.format() );  //These are all the same console.log( 'localData unix = ' + localDate.valueOf() ); console.log( 'localMoment unix = ' + localMoment.valueOf() ); console.log( 'utcMoment unix = ' + utcMoment.valueOf() );  //These formats are different console.log( 'localDate = ' + localDate ); console.log( 'localMoment string = ' + localMoment.format() ); console.log( 'utcMoment string = ' + utcMoment.format() ); console.log( 'utcDate  = ' + utcDate );  //One to show conversion console.log( 'localDate as UTC format = ' + moment.utc( localDate ).format() ); console.log( 'localDate as UTC unix = ' + moment.utc( localDate ).valueOf() ); 

Which outputs this:

localData unix = 1415806206570 localMoment unix = 1415806206570 utcMoment unix = 1415806206570 localDate = Wed Nov 12 2014 10:30:06 GMT-0500 (EST) localMoment string = 2014-11-12T10:30:06-05:00 utcMoment string = 2014-11-12T15:30:06+00:00 utcDate  = Wed Nov 12 2014 10:30:06 GMT-0500 (EST) localDate as UTC format = 2014-11-12T15:30:06+00:00 localDate as UTC unix = 1415806206570 

In terms of milliseconds, each are the same. It is the exact same point in time (though in some runs, the later millisecond is one higher).

As far as format, each can be represented in a particular timezone. And the formatting of that timezone'd string looks different, for the exact same point in time!

Are you going to compare these time values? Just convert to milliseconds. One value of milliseconds is always less than, equal to or greater than another millisecond value.

Do you want to compare specific 'hour' or 'day' values and worried they "came from" different timezones? Convert to UTC first using moment.utc( existingDate ), and then do operations. Examples of those conversions, when coming out of the DB, are the last console.log calls in the example.

Answer 2

Calling toDate will create a copy (the documentation is down-right wrong about it not being a copy), of the underlying JS Date object. JS Date object is stored in UTC and will always print to eastern time. Without getting into whether .utc() modifies the underlying object that moment wraps use the code below.

You don't need moment for this.

new Date().getTime() 

This works, because JS Date at its core is in UTC from the Unix Epoch. It's extraordinarily confusing and I believe a big flaw in the interface to mix local and UTC times like this with no descriptions in the methods.