JavaScript中星期几不正确

Question

For the following date:

Wed Jan 06 2016 02:00:00 GMT+0200 (IST) 

The following expression (which is supposed to return the day of week of January 1)

new Date(date.getUTCFullYear(), 0).getUTCDay()

returns 4 instead of the expected 5 (it was Friday...)

Why it doesn't work?

Answer 1

I think that's because new Date is not in UTC anymore while getUTCDay is. Use getDay instead.

$ new Date(2016, 0)

Fri Jan 01 2016 00:00:00 GMT+0200 (EET)

notice the +0200 part.

Answer 2

I hope this example will clarify your doubt

I am in IST time zone (GMT+0530) and depending upon the time of the day, either UTC is one day behind or the same day. Hence the result.

var d1 = new Date(2016, 0, 6, 5, 29, 0); 
var d2 = new Date(2016, 0, 6, 5, 30, 0); 
console.log(d1.getUTCDay(), d2.getUTCDay()); //logs 2 3