Round time DOWN to the nearest 5/15/30 minutes

[emarc1]

I couldn't find anything on actually rounding time intervals down to the nearest X.

This is necessary to aggregate time data.

MROUND only rounds to the nearest specified multiple (so also rounds up).

eg:
MROUND( 00:07:00, 15) = 0
MROUND( 00:08:00, 15) = 15

Really we want any time between 00:00:00 and 00:14:59 to round down to 00:00:00, any time between 00:15:00 and 00:29:59 to round down to 00:15:00, etc.