Why do the first line of code return an object of type "Date" whereas the second one returns a "number" ?

According to the doc (.asp) setHours() should just change the value of the hour of the date object, not convert it.

The code :

  var date = new Date();

  var date2 =date.setHours(19);

Thanks !

Why do the first line of code return an object of type "Date" whereas the second one returns a "number" ?

According to the doc (http://www.w3schools.com/jsref/jsref_sethours.asp) setHours() should just change the value of the hour of the date object, not convert it.

The code :

  var date = new Date();

  var date2 =date.setHours(19);

Thanks !

• javascript
• date
• typescript
• ionic2

• developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… Check out the docs. It sets the hours for the specified date, AND returns the timestamp. – CollinD Commented Dec 14, 2016 at 17:23
• developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… – Deep Commented Dec 14, 2016 at 17:23
• Look at the actual value of the date object... you'll see that it's set as you specified. – Jeff Mercado Commented Dec 14, 2016 at 17:24
• Also agree that this is a little weird they decided to do this. I stop using date and just started to use timestamps all together instead... – Endless Commented Dec 14, 2016 at 17:24
• Thanks, it was not clear in my link. Is there a way to set the hour and keep a date object ? – user5819768 Commented Dec 14, 2016 at 17:25

5 Answers 5

You need to wrap your answer in new Date(). Like this:

var date2 = new Date(date.setHours(19));

The setHours() method sets the hours for a specified date according to local time, and returns the number of milliseconds since 1 January 1970 00:00:00 UTC until the time represented by the updated Date instance.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/setHours

According to the doc (http://www.w3schools.com/jsref/jsref_sethours.asp) setHours() should just change the value of the hour of the date object, not convert it.

That document says:

Return Value: A Number, representing the number of milliseconds between the date object and midnight January 1 1970

It doesn't convert the date object though. It modifies the existing one and then returns a value.

The date object you created before still exists, with the modified hour value, and any references to it (like the one you still have in the variable named date) will remain available.

It's quite simple, just read the documentation about it.

See below what that method returns.

Return value

The number of milliseconds between 1 January 1970 00:00:00 UTC and the updated date.

I came here because I got a date.getTime is not a function error from Firebase Cloud Functions.
It is important to remember that setHours() returns a timestamp (number) AND modifies the date object (date).

For Firebase users who want to use the firebase timestamp and setHours() method:

// import Timestamp
const { Timestamp } = require('firebase-admin/firestore');

// create a new date object and reset the time to 00:00 (to sort or compare just by days, not by time/hours)
const dateForQuery = new Date();
dateForQuery.setHours(0, 0, 0, 0);

// setHours returns a timestamp, but it also modifies the date object. At this point, dateForQuery is already modified and we don't need to wrap it again in a new Date().
...
dateCreated: Timestamp.fromDate(dateForQuery),
...