Nanny Shutdown will automatically shutdown your computer after consuming a predefined time credit a day. I made this tool to temper the time my kids were playing computer games.

You can stop and start your computer as many times a day you wish. Nanny Shutdown will take into account all those different timeslots. User is informed with spoken messages during the last hour, so playing games is not interfered. Settings can be applied to the system or user(s).