How to determine the number of days in a month in SQL Server?

I need to determine the number of days in a month for a given date in SQL Server.

Is there a built-in function? If not, what should I use as the user-defined function?

Answer

You can use the following with the first day of the specified month:

datediff(day, @date, dateadd(month, 1, @date))

To make it work for every date:

datediff(day, dateadd(day, 1-day(@date), @date),
              dateadd(month, 1, dateadd(day, 1-day(@date), @date)))