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

admin管理员组

文章数量:1278948

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.

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.

• javascript
• date

Share Improve this question Follow edited Aug 30, 2018 at 14:38 benvc 15.1k44 gold badges3838 silver badges5757 bronze badges asked Apr 3, 2017 at 10:49 Devanshi SukhijaDevanshi Sukhija 16711 silver badge1010 bronze badges 5

• 1 have you done any research? – Sagar V Commented Apr 3, 2017 at 10:50
• You can refer to stackoverflow./a/42851567/3783478 – Rajesh Commented Apr 3, 2017 at 10:52
• 3 Actually, it gives the number of days in the month prior to a particular month. – David Schwartz Commented Apr 3, 2017 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 Commented Apr 3, 2017 at 10:57
• @MatteoTassinari Only if you've never used the Date constructor for anything else. ;) – David Schwartz Commented Apr 3, 2017 at 10:58

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 10

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

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

本文标签：