While you're investigating JLS' questions, this is worth playing with:
SELECT
[DTstart date],
[DTend date],
SecondsDifference = DATEDIFF(second,[DTend date],[DTstart date]),
SecondsDifference = ABS(DATEDIFF(second,[DTstart date],[DTend date]))
FROM (
SELECT
[DTstart date] = CONVERT(DATETIME,STUFF(STUFF(CAST(d.[start date] AS VARCHAR(8)),3,0,':'),6,0,':'),114),
[DTend date] = CONVERT(DATETIME,STUFF(STUFF(CAST(d.[end date] AS VARCHAR(8)),3,0,':'),6,0,':'),114)
FROM (
SELECT [start date] = '235641', [end date] = '001612'
) d
) e
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden