Difference Between Two Days And Minus A Day

Assuming the first date is in A1, and the second date is in B1, in standard dd/mm/yyyy form, my current formula is =B1-A1-1.The '-1' is due to the fact that if a patient stays for 10 days, they will only spend 9 nights in the hospital. (Bed Nights).The problem is, the formula is stretched in the total column from, say C1 to C50. Each one of these has, or will have, a number of days in it.However, due to having th '-1' in the formula, empty rows that are yet to have a patients details inputted have a -1 where I need a 0. The only reason I need to change this is because I need a running total of the bed nights of all the patients.I think the formula I'm after is something along the lines of; 'If cell B2 is empty, input 0. If B2 has a date, use formula 'B2-A2'