Changing the Contents of a Cell
 

I am currently making a spreadsheet for recording productivity via time.
To prevent errors I want to be able to have the Start Time be a copy of the
End Time on the previous line. Also due to breaks and such this might not
always be the case.

What i'd like is some way for a time to be automatically be in the Start
Time, but also be over typeable.

In the past i've just used a formula and let it be overtyped, but some of
the people that will be using this form arn't the best with computers and
this might cause problems.

So is there any way to populate a cell if the cell is blank?