Thank you for including model number.
TM01DHBL is typical analog timer with 1/2 hour segments located around edge of dial.http://waterheatertimer.org/How-to-set-timers.html
There are no repairs for defective timer: Here are suggestions.
1) Locally purchased timer has longer warranty period since you can buy same timer later and return defect in new package.
2) Top of timer has slide switch. Most analog timers have 2-or-3 position switch that offers override capability. Set some segments, move slide switch each direction, and then manually rotate timer dial clockwise two full revolutions to watch how lamp turns on-off.
If lamp does not turn on-off with switch in at least one of the slide-switch positions, then timer is suspect.
3) Always a good policy to test outlets with cheap circuit analyzer to check for electrical abnormalities that can affect timer electronics.
Since this is mechanical timer with simple electric clock motor, chances are that reversed polarity will not immediately affect timer.
4) Put timer on a different outlet, controlling a different load, and see if problem is the same.
5) Make sure segments are pulled all the way up.
Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.