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Question

I'm encountering some weird behavior while using Moment.js. I have some helper classes attached to the Date prototype, which are apparently are causing each date to display behind by a day.

Date.prototype.format = function(){
    return moment(this).format('MM/DD/YYYY')
}

var date = new Date('2008-05-13T00:00:00')

date.format() // => 05/12/2008, but should be 05/13/2008

A couple of other weird things I've noticed:

date.getDate() // => yields 12, but should be 13

But, if I instantiated the Moment object directly with the UTC string, then it works:

moment('2008-05-13T00:00:00').format('MM/DD/YY') // => 05/13/08

But I'm dealing with plain date objects, and modifying every date to a Moment object isn't my favorite idea. I have already tried modifying the format function to extract the UTC string from the date and see if it displays correctly then, but to no avail.

date.toUTCString() // => Correctly yields "Tue, 13 May 2008 00:00:00 GMT"

moment(date.toUTCString()).format('MM/DD/YY') // => still 05/12/08

Any ideas what's happening here? Is there a problem with the date constructor?

EDIT: Outputting the time as well:

moment(date).format('MM/DD/YY hh:mm:ss') // => "05/12/08 08:00:00"

Answer 1

You have to tell moment.js that you to display the date in UTC:

moment(this).utc().format('MM/DD/YYYY')

More in the documentation: http://momentjs.com/docs/#/parsing/utc/

---

But, if I instantiated the Moment object directly with the UTC string, then it works:

 moment('2008-05-13T00:00:00').format('MM/DD/YY') // => 05/13/08` 

Moment.js interprets the argument as local time :

By default, moment parses and displays in local time.

Whereas new Date() (and Date.parse ) interpret the value as UTC time :

The parse function [...] interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time.

---

I have already tried modifying the format function to extract the UTC string from the date and see if it displays correctly then, but to no avail.

 date.toUTCString() // => Correctly yields "Tue, 13 May 2008 00:00:00 GMT" moment(date.toUTCString()).format('MM/DD/YY') // => still 05/12/08` 

The format that date.toUTCString() yields is not in any of the formats that moment.js supports , so it falls back to using new Date() (which interprets the string as UTC time, not local time).