I have a problem with a cell i need to leave blank.#

A1 and A2 are formatted as times A3 will be the difference between them and A4 will show a 1 if under 5 mins.=(IFA3<L5),1,0)) L5 being 00:05
Problem is when i copy down A3 it shows 00:00 because i have no info yet in A1 and A2 How do i leave A3 blank so i dont get all 1,s

Thanks if anybody can help