Option to turn off roll over unused time from time periods that are not blocked

[Kenqr]

If I block 0900-1800 and set time limit to 20 minutes in every 3 hours, I'll have 40 minutes during 0900-1200, because of the extra 20 minutes rolled over from 0600-0900.
Since 0600-0900 is not blocked, I don't think it should roll over unused time to the next period.

After all, I don't want to have extra time to slack off in the morning, which is usually my most productive time.