The timer does not really stop.
You set the time when to post.
For example. Now is 9:00, you set posting for 15:00, it's 6 hours counting down. You close the program and open it at 14:00, one hour will be left for counter in program.
If you open the program later than 15:00, let's say at 16:00, it will ask what to do because posting time is already in the past (with options to post now or change time).
If you really to post something at night, you have two options:
1. Just leave the PC running. I personally never shut down my PC, nothing bad will happen.
2. Buy a VPS and run themaPoster there 24/7. ( just make sure to check the terms:
http://poster.freddy.lt/terms.php )