How does Parental Controls Count Time?

I set Parental Controls to allow no more than 2 hours per day, no sooner than 6:00 am and no later than 9:30 pm. Using OS x 10.6 (Snow Leopard). So far, so good. When I switch over to the controlled account for the first time in the day, say at around 7:00 pm, the controls message says that time is expired.

The problem is that the account has staid on, but there has not been ANY use of it, so I would expect all of the allowed time (2 hours in this example) to be available. I figured that time would be based on active time, not on logged-in time. I need to keep the account open inorder to allow a backup program to run.

Do I understand this correctly?

Thanks!