Returns the number of calendar days in a specified month of specified year.
year_input - the 4-digit year to be considered.
month_input - the month (in numeric format) to be considered.
DAYSINMONTH(2000, 5) equals 31, which means there are 31 calendar days in the month of May 2000.
DAYSINMONTH(1996, 2) equals 29, which means there are 29 calendar days in the month of February 1996.