days between dates, ignore blanks

[dr mint]

I am tryin to get show completion time for assignments.
i need 2 seperate formulas

1. one that will count the days it took for an item to be completed ( start date in a1 and completion date in b1) but if there is a blank in b1 then ignore count.
2. the second count the days it took for completed items (start in a1 and complete in b1) but also show the days the open items have been open meaning that b1 will be blank in some cases but i would like to know for how many days.

All sugestions and help will be appriciated.

[dr mint]

if i can achieve #2 i will be happy

[Ace_XL]

=if(b1="",today(),b1)-a1

[dr mint]

worked great thanks