Try this, you could put it in a function which accepts the time input and return the time to the next 15 minutes...
DECLARE @myDate SMALLDATETIME
SET @myDate = '12 march 2004 12:46'
SET @myDate = DATEADD(MINUTE, (DATEPART(MINUTE, @myDate) + 14) / 15 * 15 - DATEPART(MINUTE,@myDate), @myDate)...