I personaly cringe when I see manual calculations involving date/time. It's way to easy to mess up and there are usualy good library functions for it.

In T-SQL would I for instance do something like this

SELECT

DATEPART(

dd,

DATEADD(

dd,

-1,

DATEADD(

mm,

1,

CONVERT(varchar(8), @Date, 20) + '01'

)

)

) AS [DaysInMonth]

to get the numer of days in the current month. That way are you safe with all leap years and don't have to worry about a thing.