I am a trying to create a Javascript date object from a time in milliseconds computed from GMT0 (or UTC).
I use the following code for a time located in 2013 (as verifiable here):
var t = new Date(Date.UTC(0, 0, 0, 0, 0, 0, 0));
t.setMilliseconds(1383447600000);
but when I call the following:
alert(t.getFullYear());
alert(t.getUTCFullYear());
I am getting 1943... and not 2013!
Why? And how to solve this? Thanks!
The JsFiddle is: http://jsfiddle.net/EVf72/
Short Answer: Use setTime instead of setMilliseconds.
Long Answer:
The problem is that your starting date is incorrect. The value of 1383447600000 is the number of seconds since epoch 0 (January 1, 1970, 00:00:00 UTC), but your starting date is not epoch 0! Instead, it is the year 1899:
> var t = new Date(Date.UTC(0, 0, 0, 0, 0, 0, 0));
> console.log(t.getFullYear());
1899
When you then use setMilliseconds and provide a range over 999, it will convert the value into the appropriate numbers of years, days, hours, seconds, and milliseconds and add it to the current date.
1383447600000 corresponds to a little over 43 years. So you're basically telling JavaScript to add a little over 43 years to 1899, which gives you 1943.
From the documentation for setMilliseconds:
If you specify a number outside the expected range, the date information in the Date object is updated accordingly. For example, if you specify 1005, the number of seconds is incremented by 1, and 5 is used for the milliseconds.
If you had instead provided the correct starting point to Date.UTC so that it matches epoch 0, you would have received the correct answer:
> var t = new Date(Date.UTC(1970, 0, 0, 0, 0, 0, 0)); //First param is year
> t.setMilliseconds(1383447600000);
> console.log(t.getFullYear());
2013
But instead of doing all of that, you can simply use setTime:
> var t = new Date();
> t.setTime(1383447600000);
> console.log(t.getFullYear());
2013
So to recap, the following are functionally equivalent:
> var t = new Date(Date.UTC(1970, 0, 0, 0, 0, 0, 0)); //First param is year
> t.setMilliseconds(1383447600000);
> console.log(t.getFullYear());
2013
and
> var t = new Date();
> t.setTime(1383447600000);
> console.log(t.getFullYear());
2013
But if you are dealing with milliseconds since epoch 0, you either need to use setTime, or make sure that you actually start with epoch 0 (using Date.UTC) if you are going to be using setMilliseconds.