I've always thoughts that BizTalk will start counting when the host instance gets restarted or the ports enabled. And when the interval hits inside the service window, it will do whatever it needs to do.
We did a test today and I was surprised. BizTalk actually executes at the start of the service window and counts the interval time from that point.
We were going to set up a SQL receive location polling every 24 hours to execute a daily task, but with this new discovery, we are going to set up a receive location with the polling interval longer than the service window. Advantage? We can ensure that the start time of the process is always consistent, baring any SQL server downtime.