I do not have a manual for GE 51583 timer.
http://waterheatertimer.org/GE-timers-and-manuals.html#51583Generally speaking, programmable timers are set one time, and then they
repeat that program day after day until timer malfunctions or you reset
programming.
If your timer is resetting itself, then electricity might be going off, for example a loose wire?
Try timer on another outlet.
Test outlet with cheap circuit analyzer from home center.
It is hard to answer question without guessing until I can read the manual.
If you have instruction manual, I would enjoy getting a copy.
Scan manual page(s) into jpeg/jpg format to your computer and then add a comment with those photos so I can see instructions.
http://www.fixya.com/support/r6568559-post_photograph_fixya_questions_answersAdd a comment with your experience so others with same-similar problem can benefit.
Also 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.