javascript to find leap year

Question

How can I get the code below to work when I have a month of february? Currently it is getting to the day and then stopping before getting to the if to determine whether it is a leap year.

 if (month == 2) {     if (day == 29) {         if (year % 4 != 0 || year % 100 == 0 && year % 400 != 0) {             field.focus();              field.value = month +'/' +  '';         }     }     else if (day > 28) {         field.focus();              field.value = month +'/' +  '';     } } 

Answer 1

It's safer to use Date objects for datetime stuff, e.g.

isLeap = new Date(year, 1, 29).getMonth() == 1 

Since people keep asking about how exactly this works, it has to do with how JS calculates the date value from year-month-day (details here). Basically, it first calculates the first of the month and then adds N -1 days to it. So when we're asking for the 29th Feb on a non-leap year, the result will be the 1st Feb + 28 days = 1st March:

> new Date(2015, 1, 29) < Sun Mar 01 2015 00:00:00 GMT+0100 (CET) 

On a leap year, the 1st + 28 = 29th Feb:

> new Date(2016, 1, 29) < Mon Feb 29 2016 00:00:00 GMT+0100 (CET) 

In the code above, I set the date to 29th Feb and look if a roll-over took place. If not (the month is still 1, i.e. February), this is a leap year, otherwise a non-leap one.

Answer 2

Compared to using new Date() this is is around 100 times faster!

Update:

This latest version uses a bit test of the bottom 3 bits (is it a multiple of 4), as well as a check for the year being a multiple of 16 (bottom 4 bits in binary is 15) and being a multiple of 25.

ily = function(y) {return !(y & 3 || !(y % 25) && y & 15);}; 

http://jsperf.com/ily/15

It is slightly faster again than my previous version (below):

ily = function(yr) {return !((yr % 4) || (!(yr % 100) && (yr % 400)));}; 

http://jsperf.com/ily/7

It is also 5% faster, compared to the already fast conditional operator version by broc.seib

Speed Test results: http://jsperf.com/ily/6

Expected logic test results:

alert(ily(1900)); // false alert(ily(2000)); // true alert(ily(2001)); // false alert(ily(2002)); // false alert(ily(2003)); // false alert(ily(2004)); // true alert(ily(2100)); // false alert(ily(2400)); // true