Concatenating Time and DateTime in where clause - sql-server

Question

I am trying to combine a SmallDateTime field and a Time value (result of a scalar-valued function) into a DateTime and I keep getting the following error:
Conversion failed when converting date and/or time from character
string.
Here are the variables used throughout:
DECLARE #STARTDATETIME AS DATETIME
DECLARE #ENDDATETIME AS DATETIME
SELECT #STARTDATETIME = '8/29/2016 12:00:00'
SELECT #ENDDATETIME = '8/30/2016 12:00:00'
Column definitions:
FT_START_DATE SmallDateTime
FT_END_DATE SmallDateTime
FT_START_TIME Int
FT_END_TIME Int
The date fields do not contain timestamps. The time fields are basically 24 hour time without the colon dividers. (Example: 142350 = 14:23:50)
Here's the function that is called in my queries:
USE [PWIN171]
GO
/****** Object: UserDefinedFunction [dbo].[dbo.IPC_Convert_Time] Script Date: 9/13/2016 4:50:49 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[dbo.IPC_Convert_Time]
(
#time int
)
RETURNS time
AS
BEGIN
DECLARE #Result time
SELECT #Result = CONVERT(time
, STUFF(
STUFF(
RIGHT('000000' + CONVERT(varchar(6), #time), 6)
, 5, 0, ':')
, 3, 0, ':')
)
RETURN #Result
END
Example 1 - Fails:
This is what I'm after in general.
SELECT * FROM FT WITH (NOLOCK)
WHERE
CAST(FT_END_DATE AS DATETIME) + DBO.[dbo.IPC_Convert_Time](FT_END_TIME) BETWEEN #STARTDATETIME AND #ENDDATETIME;
Example 2 - Works:
This one runs, but it won't get records from 8/29 because the end dates will be before 12:00:00 on 8/29.
SELECT * FROM FT WITH (NOLOCK)
WHERE
FT_END_DATE BETWEEN #STARTDATETIME AND #ENDDATETIME
AND CAST(FT_END_DATE AS DATETIME) + DBO.[dbo.IPC_Convert_Time](FT_END_TIME) BETWEEN #STARTDATETIME AND #ENDDATETIME;
I suppose I could do one where I split apart my paramters and check that the end time is between the time portion of the parameters as well, but that seems to be a step in the wrong direction. The error seems to only appear when there is no other usage of FT_START_DATE or FT_END_DATE in the where clause.
The time converting function works fine in every scenario I have created. I have even tried Example 2 with parameters that would give it 100% overlap with the data covered by Example 1 in case there was bad data causing the error, but it runs fine.
I also don't know exactly where the error is occurring, because it only references the line the select statement begins on, and not the actual location in the code.
Why does it behave like this?
UPDATE:
TIMEFROMPARTS is not available because this is on SQL Server 2008

Answer 1

If I understand this correctly, this can be done much simpler:
Try this:
DECLARE #d DATE=GETDATE();
DECLARE #t TIME=GETDATE();
SELECT #d;
SELECT #t;
SELECT CAST(#d AS datetime)+CAST(#t AS datetime);
A pure date and a pure time can simply be added to combine them...
UPDATE Read your question again...
Try this
SELECT FT_END_DATE
,FT_END_TIME
,CAST(FT_END_DATE AS DATETIME) + DBO.[dbo.IPC_Convert_Time](FT_END_TIME) AS CombinedTime
,*
FROM FT
to see if your attempt is doing the right thing.
If yes, it might help to create a CTE and do the filter on the named column.
Sometimes the engine does not work the order you would expect this.
As CTEs are fully inlined it is quite possible, that this will not help...
SQL Server is well knwon for bringing up such errors, because a type check happens before a conversion took place...
It might be an idea to use the given SELECT with INTO #tbl to push the result set into a new table and do your logic from there...