any additional date_time functions?

I am working with the date_trunc() function with great success especially in the group by clause
for aggregates.

However, it is limited to returning "WHOLE" time units. i.e. years, months, days, hours, minutes,
seconds.

Are there any functions similar to date_trunc that can return variable increments i.e.:
5, 10, or 15 minutes increments,
3, 4, 6 hour increments,
1, 2 weekly increments,

I imagine that the returned values would have to either be the "floor" or "ceiling" of the actual
time stamps.

Regards,

Richard Broersma Jr.