How can I round up the time to the nearest X minutes?

Is there a simple function for rounding UP a DateTime to the nearest 15 minutes?

E.g.

2011-08-11 16:59 becomes 2011-08-11 17:00

2011-08-11 17:00 stays as 2011-08-11 17:00

2011-08-11 17:01 becomes 2011-08-11 17:15