I need a formula to calculate days between dates if before each other i.e.

18 Apr & 9 Apr = - 9 days

I can calculate the difference for say
13 Mar & 18 Apr = 36 days
but need a formulae to calculate both in a list
Col 1 col 2 col 3
13 Mar 18 Apr 36
18 Apr 9 Apr ?
3 Mar 30 Mar 27

Thanks any help would be appreciated