Trying to deploy Write UDF in DDT

A simple UDF, that I need to return a date based on some info in my DB, I wanted this as a UDF, which I have written and tested in SQL Server, works fine.

What I can't do is get the DDT to deploy it. I know my issue lies in the first 2-3 lines, but I really don't know what it might want.

CREATE FUNCTION [dbo].[FirstDayHolidayYear]()

RETURNS smalldatetime

BEGIN

-- Declare the return variable here

DECLARE @Year CHAR(4)

DECLARE @Month VARCHAR(2)

DECLARE @Day VARCHAR(2)

SELECT @year = YEAR(GETDATE())

SELECT @day = '01'

IF (SELECT annualleaveyearstart FROM dbo.ApplicationSettings)<10

SELECT @Month = '0'+CAST(annualleaveyearstart AS CHAR(1)) FROM dbo.ApplicationSettings

ELSE

SELECT @month = CAST(annualleaveyearstart AS CHAR(2)) FROM dbo.ApplicationSettings

BEGIN

-- Insert statements for procedure here

return (CAST(@year+'-'+@month+'-'+@day AS SMALLDATETIME))

END