Transact SQL :: Select Statement Not Working In User Defined Function

The select statement:

SELECT DATEDIFF(n , LAG(CAST(Date AS DATETIME) + CAST(Time AS DATETIME), 1) OVER ( ORDER BY Date, Time ),
       CAST(Date AS DATETIME) + CAST(Time AS DATETIME))
   FROM [DataGapTest]

Gives the right output:

NULL
1
1
3548
0

However, when I put the statement in a function, I get only zeros as the output. It's as if the lag and current value are always the same (but they are not of course).

CREATE FUNCTION dbo.GetTimeInterval(@DATE date, @TIME time)
RETURNS INT
AS
  BEGIN
 DECLARE @timeInterval INT
   SELECT @timeInterval = DATEDIFF(n , LAG(CAST(@Date AS DATETIME) + CAST(@Time AS DATETIME), 1) OVER ( ORDER BY Date, Time ),
       CAST(@Date AS DATETIME) + CAST(@Time AS DATETIME))
   FROM dbo.[DataGapTest]
  RETURN @timeInterval
  END