This might sound hugely amateurish but I have a spreadsheet for my Pilot Log Book as I am a student. I have boxes stating my Take Off time and Landing time. In this case:
13:40 (H5) to 14:40 (I5)
This means I would put down 1:00 (K5) for the amount flown.
What formula would I input to say "I5 - H5 =", but if there is no value in I5 or H5, leave K5 blank?