SQL SERVER – UDF – User Defined Function – Get Number of Days in Month

Following User Defined Function (UDF) returns the numbers of days in month. It is very simple yet very powerful and full proof UDF.
CREATE FUNCTION [dbo].[udf_GetNumDaysInMonth] ( @myDateTime DATETIME )
RETURNS INT
AS
BEGIN
DECLARE @rtDate INT
SET @rtDate = CASE WHEN MONTH(@myDateTime)
IN (1, 3, 5, 7, 8, 10, 12) THEN 31
WHEN MONTH(@myDateTime) IN (4, 6, 9, 11) THEN 30
ELSE CASE WHEN (YEAR(@myDateTime) % 4 = 0
AND
YEAR(@myDateTime) % 100 != 0)
OR
(YEAR(@myDateTime) % 400 = 0)
THEN 29
ELSE 28 END
END
RETURN @rtDate
END
GO

Run following script in Query Editor:
SELECT dbo.udf_GetNumDaysInMonth(GETDATE()) NumDaysInMonth
GO

ResultSet:
NumDaysInMonth
———————–
31

Testing the Days in Month Function

The leap year rule in this function is the full one, and that is what makes it reliable. A year is a leap year if it can be divided by 4, except years that can be divided by 100, unless they can also be divided by 400. So 2000 was a leap year, but 1900 was not.

Whenever I write a date function, I test it with a short list of dates that cover the tricky cases:

• February 2008 should return 29, and February 2007 should return 28.
• February 2000 should return 29, and February 1900 should return 28.
• April should return 30, and December should return 31.
• Try a NULL too. This version returns 28 for NULL, because every test in the CASE fails and it falls through to the last ELSE. Add a check at the top if that matters to you.

There is also a shorter way that lets SQL Server do the calendar math. Find the last day of the month, then take its day number: DAY(DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, @date) + 1, 0))). On SQL Server 2012 and later it gets even simpler with DAY(EOMONTH(@date)).

Keep in mind that a scalar function runs once for every row. For a report over a few thousand rows it is fine, but in a big query I would put the expression inline, or join to a calendar table that already stores the number of days for each month.

A calendar table is worth building once. It is a plain table with one row per date and columns for everything you ever need to know about that date: the month name, the week number, the number of days in that month, whether it is a weekend or a holiday. Queries join to it instead of calling functions, which keeps them simple and fast.

Published by Pinal Dave on SQLAuthority. More of my work at pinaldave.com.