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.

Clangor

have you done any research? – Vacla

You can refer to https://mcmap.net/q/1480269/-javascript-validate-date-and-return-last-available-date-of-calendar – Recurved

Actually, it gives the number of days in the month prior to a particular month. – Ragsdale

@DavidSchwartz that is correct, however, considering how javascript Date enumerates months, it seems like it gives the length of specified month. – Relume

@MatteoTassinari Only if you've never used the Date constructor for anything else. ;) – Ragsdale

Answer 1

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

Ragsdale

Answer 2

If any parameter overflows its defined bounds, it "carries over". For example, if a monthIndex greater than 11 is passed in, those months will cause the year to increment; if a minutes greater than 59 is passed in, hours will increment accordingly, etc. Therefore, new Date(1990, 12, 1) will return January 1st, 1991; new Date(2020, 5, 19, 25, 65) will return 2:05 A.M. June 20th, 2020.

Similarly, if any parameter underflows, it "borrows" from the higher positions. For example, new Date(2020, 5, 0) will return May 31st, 2020.

MDN

Ploy