Ok, this is a difficult problem because I do not want to suggest an expensive repair like the timer and then have that not be the problem. It is probably the timer, but lets do a few more checks.
1. Turn on the dryer and place the timer in cool down area (last 10 minutes) and see if the timer advances.
2. With machine unplugged, check timer motor for continuity.
Post the results and let's see what that tells us.