I can’t speak about what I do in office. (ok, not yet). But I need to write something. So here goes:

A “small” algo to retrieve number of days in a month. Don’t ask me it s efficiency (modulus is never fast , an alternative would be something similar to isInt function ):

/// January = 1, February = 2 and so on.

/// year will be used only if month = 2

function daysInTheMonth(var monthNumber, var yearNumber)

{

// if February, and year is divisible by 4 but not divisible by 100, return 28. Else return 28.

// since years which are divisible by 100 should not be leap years

if(monthNumber == 2) { return (yearNumber%4 ==0 && yearNumber%100>0)? 29 : 28 }

return (monthNumber > 7) ? ((monthNumber%2 ==0) ? 30 : 31) : ((monthNumber%2 ==0) ? 31: 30);

// another option

// if(monthNumber > 7) monthNumber++;

// return ((monthNumber%2 ==0) ? 30 : 31)

}