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)