The timer lets you adjust the preset astronomic time for your area.
This is because actual local times vary.
If the preset time for dusk is 8:00pm and you add 12 minutes, then the timer will turn off-or-on at 8:12.
And then you can change the preset time for dawn by adding or subtracting.
If this is not what exactly you are asking, then set your timer and see what happens, and then add a comment with your findings.
Open following link to download on-line manual
http://waterheatertimer.org/Leviton-timers.htmlAdd 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.