Number of days in any month [duplicate]

Answer 1

try this:

function DaysinMonth(aDate)  {
    return aDate.setMonth(aDate.getMonth()+1, 0).getDate();
}    

Answer 2

Try this:

function daysInMonth(year, month) {
    var isLeap = ( (!(year % 4)) && ( (year % 100) || (!(year % 400)) ) );

    if (month == 2)
        return (isLeap) ? 29 : 28;
    return 30 + (month % 2);
}

Answer 3

Loved James' answer as well as Bruno's explanation of it. However, got annoyed at the overly cryptic nature of the solution. So here is the same solution but cleaned of any unnecessary over encryption.

function getDaysInMonth(m, y) {
   return /4|6|9|11/.test(m)?30:m==2?(!(y%4)&&y%100)||!(y%400)?29:28:31;
}

Specifics:

1. Seems there's no need to decrease month - javascript has nothing to do with it, since we only use it for comparison, so I used the real month numbers for clarity.

2. Why write the number of april, june, september and november out of order? That's just confusing.

3. *Optionally, we can increase the month (++m) to get a version that accepts (new Date()).getMonth() as input

Answer 4

function getDaysInMonth(m, y) {
   return /8|3|5|10/.test(--m)?30:m==1?(!(y%4)&&y%100)||!(y%400)?29:28:31;
}

Answer 5

Loved James' answer. Reformatted slightly for those interested.

function getDaysInMonth(m, y)
{
    // months in JavaScript start at 0 so decrement by 1 e.g. 11 = Dec
    --m;

    // if month is Sept, Apr, Jun, Nov return 30 days
    if( /8|3|5|10/.test( m ) ) return 30;

    // if month is not Feb return 31 days
    if( m != 1 ) return 31;

    // To get this far month must be Feb ( 1 )
    // if the year is a leap year then Feb has 29 days
    if( ( y % 4 == 0 && y % 100 != 0 ) || y % 400 == 0 ) return 29;

    // Not a leap year. Feb has 28 days.
    return 28;
}

Fiddle here