Sql-server – Different Results between stored procedure and UDF

I am getting different results between my stored procedure and my UDF.

The jist of of the both of them is that, it queries a remotely linked server, jams those results into a temp table/table variable, then replaces the NULLs with '00:00:00' so that I can sum the amounts. Then takes that date, and does a DATEDIFF on it to give me the total hours worked.

If I input the values ('John Doe', '04/05/2013', '04/16/2013'), the stored procedure is returning the answer I am looking for, the SVF is not. Where the stored procedure would result in "23" for hours worked, the SVF returns "1900-01-24 00:00:00".

I realize that it is with what I am returning in the SVF, but I can't figure it out. I have tried to return TIME(0), VARCHAR(5). It is as if the SVF isn't doing the DATEDIFF bit.

I am working with SQL Server 2012 (11.0.3000)

Below is the SVF and the stored procedure.

The SVF

ALTER FUNCTION [dbo].[fnFindHoursWorked] 
    (@Name varchar(50)
     ,@Start_Date DATE = NULL
     ,@End_Date DATE = NULL)
RETURNS DATETIME
AS
BEGIN
    -- Declare the return variable here
    DECLARE @HoursWorked DATETIME

    -- Add the T-SQL statements to compute the return value here
    /** create a table variable to store the results of the query **/
    DECLARE @Duty TABLE (Time_on_Duty TIME(0))

    INSERT INTO @Duty(Time_on_Duty)

    /** The actual query that grabs the data from the  server **/
    SELECT   
        CAST(UL.[Date_Time] - LAG(UL.[Date_Time], 1) OVER (PARTITION BY CAST(UL.[Date_TIME] AS DATE) ORDER BY UL.[Date_TIME]) AS TIME(0)) AS 'Time On Duty'
    FROM [LinkedServer].[database].dbo.[tablename] AS UL
    WHERE UL.[Department] = 'Department'
      AND ((UL.[Action] = 'OnDuty' OR UL.[Action] = 'Login') OR UL.[Action] = 'OffDuty')
      AND (UL.[Name] = @Name)
      AND ((CAST(UL.[Date_TIME] AS DATE) >= @Start_Date AND @End_Date IS NULL)
        OR (CAST(UL.[Date_TIME] AS DATE) <= @End_Date AND @Start_Date IS NULL)
        OR (CAST(UL.[Date_TIME] AS DATE) >= @Start_Date AND CAST(UL.[Date_TIME] AS DATE) <= @End_Date)
            )

    /** Setting all of the NULLS in the table variable to a value on which we can do math **/
    UPDATE @Duty 
    SET Time_on_Duty = '00:00:00'
    WHERE Time_on_Duty IS NULL

    /** The select statement to grab the total hours worked for the date range **/
    SET @HoursWorked = (SELECT DATEDIFF(hour,'1900-01-01 00:00:00',CAST(SUM(CAST(CAST(Time_on_Duty AS DATETIME) AS FLOAT)) AS DATETIME))  AS 'Time_On_Duty' FROM @Duty)

    -- Return the result of the function
    RETURN @HoursWorked
END

Here's the stored procedure:

ALTER PROCEDURE [dbo].[rptTotal_Time_On_duty] 
    @Name varchar(50)
    ,@Start_Date DATE = NULL
    ,@End_Date DATE = NULL
AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    -- Insert statements for procedure here
    IF OBJECT_ID (N'tempdb..#Duty', N'U') IS NOT NULL
       TRUNCATE TABLE #Duty
    ELSE
       CREATE TABLE #Duty (Time_on_Duty TIME)

    INSERT INTO #Duty(Time_on_Duty)
        SELECT   
            CAST(UL.[Date_Time] - LAG(UL.[Date_Time],1) OVER (PARTITION BY CAST(UL.[Date_TIME] AS DATE) ORDER BY UL.[Date_TIME]) AS TIME) AS 'Time On Duty'
        FROM [LinkedServer].[database].dbo.[tablename] AS UL
        WHERE UL.[Department] = 'Department'
          AND ((UL.[Action] = 'OnDuty' OR UL.[Action] = 'Login') OR UL.[Action] = 'OffDuty')
          AND (UL.[Name] = @Name)
          AND (
           (CAST(UL.[Date_TIME] AS DATE) >= @Start_Date AND @End_Date IS NULL)
        OR (CAST(UL.[Date_TIME] AS DATE) <= @End_Date AND @Start_Date IS NULL)
        OR (CAST(UL.[Date_TIME] AS DATE) >= @Start_Date AND CAST(UL.[Date_TIME] AS DATE) <= @End_Date)
        )

    UPDATE #Duty 
    SET Time_on_Duty = '00:00:00'
    WHERE Time_on_Duty IS NULL

    SELECT DATEDIFF(hour,'1900-01-01 00:00:00',CAST(SUM(CAST(CAST(Time_on_Duty AS DATETIME) AS FLOAT)) AS DATETIME))  AS 'Time_On_Duty'
    FROM #Duty

END