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javascriptc#datedatetimemomentjs
Raúl Otaño
Raúl Otaño

Reputation: 4770

Moment date issue when manipulating dates

All the problem starts when I detect an odd behavior: I had a long number that means "the number of milliseconds since 0001/01/01", then in C# date time when I use the AddMilliseconds get a different date value that the one returned from the moment one, in one hour different. eg.

new DateTime().AddMilliseconds(63613091700000);  =>  {10/26/2016 3:15:00 PM}
moment([1]).add(63613091700000).toDate()         =>  Wed Oct 26 2016 16:15:00 GMT-0400 (Eastern Daylight Time)

In C# get the 15:15h and in the moment 16:15!!!

That blown my mind, so I search the error, and I find it:

moment([1]).toDate()   =>  Mon Jan 01    1 00:00:00 GMT-0500 (Eastern Standard Time)

The problem was that when I create the moment from a custom date (moment([1])) it use the GMT-0500 (Eastern Standard Time) but when apply the moment add method it returns GMT-0400 (Eastern Daylight Time)! Also check that creating the moment date by moment() or using the javascript date new Date() also use the GMT-0400 (Eastern Daylight Time). So that is the problem.

My question is, why does this happen? Is it an issue?

Upvotes: 0

Views: 907

Answers (2)

RobG
RobG

Reputation: 147483

ECMAScript treats daylight saving rules as if they always applied, even before daylight saving was introduced.

When you create a Date for 0001-01-01 in host set to US EST time zone, there was no daylight saving (because US EDT runs from March to early November) so a UTC time value is calculated for 0001-01-01T00:00:00-0500 or 0001-01-01T05:00:00Z (they are the same moment in time).

On the same host, when you add enough milliseconds to get to 2016-10-26 15:15:00-0500 in a place that has daylight saving on that day, then the time will increment 1 hour because of the different offset for daylight saving, so the time is shown as 16:15:00 or in full: 2016-10-26T16:15:00-0400.

But note that 2016-10-26T15:15:00-0500 and 2016-10-26T16:15:00-0400 are exactly the same moment in time (and equivalent to 2016-10-26T20:15:00Z), the only difference is the timezone offset.

If you want to display dates and times for a particular timezone other than UTC, use moment-timezone.js.

Upvotes: 1

Raúl Otaño
Raúl Otaño

Reputation: 4770

I have found what is happening, but not how to handle it: The problem is the Is Daylight Saving Time of javascript.

From http://momentjs.com/docs/#/query/is-daylight-saving-time/:

moment([2011, 2, 12]).isDST(); // false, March 12 2011 is not DST
moment([2011, 2, 14]).isDST(); // true, March 14 2011 is DST

The thing is that when I do moment([1]).add(63613091700000).toDate() it calculates in the right way, but the result datetime is in DST so it transform it. I doesn't want that behavior this time.

Upvotes: 0

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