Subtract minute from DateTime in SQL Server 2005

Question

Suppose I have a datetime field whose value is 2000-01-01 08:30:00 and a duration field whose value is say 00:15 (meaning 15 minutes)

If I subtract these two, I should get 2000-01-01 08:15:00

Also if I want to subtract 1:15 (means 1 hour 15 minutes), the output should be 2000-01-01 07:15:00

I am trying SELECT DATEDIFF(minute, '00:15','2000-01-01 08:30:00');

But the output is 52595055. How can i get the desired result?

N.B.~ If I do SELECT dateadd(minute, -15,'2000-01-01 08:30:00'); , I will get the desired result but that involves parsing the minute field.

Edit:

As per the answers, every one is suggesting converting everything into minutes and then to subtract - so if it is 1:30, i need to subtract 90 minutes. That's fine. Any other way without converting to minutes?

Answer 1

SELECT DATEADD(minute, -15, '2000-01-01 08:30:00');  

The second value (-15 in this case) must be numeric (i.e. not a string like '00:15'). If you need to subtract hours and minutes I would recommend splitting the string on the : to get the hours and minutes and subtracting using something like

SELECT DATEADD(minute, -60 * @h - @m, '2000-01-01 08:30:00');  

where @h is the hour part of your string and @m is the minute part of your string

EDIT:

Here is a better way:

SELECT CAST('2000-01-01 08:30:00' as datetime) - CAST('00:15' AS datetime)