Rounding Time in Increments

I Recently needed to round down a time in certain increments. This is not something I've ever been asked to do previously so it was an interesting problem to figure out. The resulting formula is simple:

TIME_MIN - (TIME_MIN % INCREMENT)

This will give you the previous incremental minute. ex: Given a 5 minute increment @ 3:12, the result would be 10.

time = Time.now
new_min = time.min - (time.min % 5)

I happen to be solving this in a project with ActiveSupport so there was access to Time#change so I finished this up with:

time.change(min: new_min)