Number of days in a month in Numbers

And a third way that, given any date, calculates the number of days in that month.

Thus, if you fill the month names down the column and Numbers uses its smart date recognition to store them as dates, then this also works:

=DAY(DATE(YEAR(A1),MONTH(A1)+1,1)−1)

SG

Here are two ways:

Method 1 requires the "Month Num" column (which makes the formula a little easier to read):

cell C1 contains the year (since the number of days changes by year).

rows 1 and two are header rows

C3=EOMONTH(DATE($C$1, B3, 1),0)−DATE($C$1, B3, 1)

this is shorthand for... select cell C3, then type (or copy and paste from here) the formula:

=EOMONTH(DATE($C$1, B3, 1),0)−DATE($C$1, B3, 1)

select cell C3, copy

select cells C3 thru C14, paste

If you do not want to have the "Month Num" column (column B) you can use Method 2:

D3=EOMONTH(DATE($C$1, ROW()−2, 1),0)−DATE($C$1, ROW()−2, 1)

fill down like before

the function EOMONTH() returns the End Of MONTH date. the subtraction of two dates is a duration.