Why does JavaScript: new Date(year, month, 0).getDate() return the number of days in the month?

Question

I know that this little javascript code,var whatever = new Date(year, month, 0).getDate(), returns the number of days in a particular month of a particular year. But what I don’t seem to understand is the logic behind it.

What exactly is that zero doing there after we mention the year and the month? Please Explain.

You can refer to http://stackoverflow.com/a/42851567/3783478 — Rajesh, Apr 03 '17 at 10:52
Actually, it gives the number of days in the month *prior* to a particular month. — David Schwartz, Apr 03 '17 at 10:55
@DavidSchwartz that is correct, however, considering how javascript Date enumerates months, it *seems* like it gives the length of specified month. — Matteo Tassinari, Apr 03 '17 at 10:57
@MatteoTassinari Only if you've never used the `Date` constructor for anything else. ;) — David Schwartz, Apr 03 '17 at 10:58

score 9 · Accepted Answer · answered Apr 03 '17 at 10:57

When you give a parameter out of range, the next larger time increment is adjusted to make the time valid. So:

> new Date(2016,2,1)
2016-03-01T08:00:00.000Z

So if we specify (2016,2,1), we get 3/1. So if we specify (2016,2,0), we'll get a day before that, adjusting the month as necessary to get something valid, that is, the last day of the previous month.

> new Date(2016,2,0)
2016-02-29T08:00:00.000Z

Why does JavaScript: new Date(year, month, 0).getDate() return the number of days in the month?

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