Sequence Starting and Ending Time

There seems to be a couple of similar requests here but what I’m looking for is to have an adjusted starting and ending time based on my input and the current date. For example, based on my visibility the target gets over the trees at 10 pm tonight and in two weeks the target is visible for me 10 minutes earlier. Now I have to adjust the starting time manually so what would it need to make this adjustment an automatic one?