You can use this function

CREATE FUNCTION dbo.fnGetMonthDays(@myDate DATE) RETURNS INT

AS

BEGIN

DECLARE @isLeap INT = 0

IF (YEAR(@myDate) % 400 = 0 OR (YEAR(@myDate) % 4 = 0 AND YEAR(@myDate) % 100 !=0))

SET @isLeap=1

DECLARE @month INT = MONTH(@myDate)

DECLARE @days INT

SELECT @days =

CASE

WHEN @month=1 THEN 31

WHEN @month=2 THEN 28 + @isLeap

WHEN @month=3 THEN 31

WHEN @month=4 THEN 30

WHEN @month=5 THEN 31

WHEN @month=6 THEN 30

WHEN @month=7 THEN 31

WHEN @month=8 THEN 31

WHEN @month=9 THEN 30

WHEN @month=10 THEN 31

WHEN @month=11 THEN 30

WHEN @month=12 THEN 31

END

RETURN @days

END

SELECT dbo.fnGetMonthDays(CONVERT(DATE,'2014-02-01'))

--28

SELECT dbo.fnGetMonthDays(CONVERT(DATE,'2016-02-01'))

--29