Wouldn't you just count back 30 minutes from 10:08?
If you round up to 10:10, it's a little easier. Picture an analog clock in your head. Every big number is five minutes, and every two is ten minutes. If we put it back to the 12, that's ten minutes back. Twenty minutes earlier from that would be 9:40. Since we added two minutes to make it easier, we've got to subtract those two minutes. So, the starting time would be 9:38.
I hope this isn't too confusing.