TSQL need to return last day of month only, how do I drop year, month & time?

I am writing a function in T-SQL returning the last day of the month regardless of the date input.

Here is my code:

Alter Function dbo.FN_Get_Last_Day_in_Month2
(@FN_InputDt    Datetime)
Returns smalldatetime
as 
Begin

Declare @Result  smalldatetime
Set    @Result =

       case when @FN_InputDt <> 01-01-1900 then 
       DATEADD(m, DATEDIFF(M, 0,@FN_InputDt)+1, -1)
       Else 0 End

Return @Result

End 

The code is not working correctly, here is a test that shows the bad behavior:

SELECT dbo.fn_get_last_day_in_month (07-05-2010)

Here is the (incorrect) result:

2010-07-31 00:00:00

What is 07-05-2010...May 7th or July 5th? You need to use a safe date format, take a look at Setting a standard DateFormat for SQL Server

example from How to find the first and last days in years, months etc

DECLARE @d DATETIME
SET @d = '20100705'  -- notice ISO format

SELECT
    DATEADD(yy, DATEDIFF(yy, 0, @d), 0) AS FirstDayOfYear,
    DATEADD(yy, DATEDIFF(yy, 0, @d)+1, -1) AS LastDayOfYear,
    DATEADD(qq, DATEDIFF(qq, 0, @d), 0) AS FirstDayOfQuarter,
    DATEADD(qq, DATEDIFF(qq, 0, @d)+1, -1) AS LastDayOfQuarter,
    DATEADD(mm, DATEDIFF(mm, 0, @d), 0) AS FirstDayOfMonth,
    DATEADD(mm, DATEDIFF(mm, 0, @d)+1, -1) AS LastDayOfMonth,
    @d - DATEDIFF(dd, @@DATEFIRST - 1, @d) % 7 AS FirstDayOfWeek,
    @d - DATEDIFF(dd, @@DATEFIRST - 1, @d) % 7 + 6 AS LastDayOfWeek

for just the day use day or datepart

 select DAY(getdate()),
     DATEPART(dd,GETDATE())

Cast the return value to a SQL datetime type, and then call the "DAY" function to get the day in as an integer. See the function reference here:

http://msdn.microsoft.com/en-us/library/ms176052.aspx

Not sure which database you're using, but this should be a standard function across all databases.

I'd return a DATETIME, I've had trouble with SMALLDATETIME in the past.

DECLARE @Result DATETIME

SET @Result = DATEADD(m , 1, @FN_Input);

RETURN CAST(FLOOR(CAST(DATEADD(d, DATEPART(d, @Result) * -1, @Result) AS FLOAT)) AS DATETIME)

Also, I think you may be a victim of SQL's complete disregard of date formatting. Always, always, always, when typing a string into test a SQL function use the following format;

'05 Jul 2010'

Your function probably works but it interpreted your date as 5th July - not 7th May.

DECLARE @date DATETIME = '20130624';
SELECT Day(EOMONTH ( @date )) AS LastDay;
GO