I'm a beginner...Please help

Dates are inputted in column M or H; but sometime dates are not required in
M and H which would leave it blank (if date is in column M there will not be
a date in column H and vice versa);

Cell B1 should always prompt today's date (=today())

I need column N to calculate days between them
example: B1=05/08/08 M=05/21/08 N should = 13
But if there's nothing in M and H it returns -39576
In that case I need "0" to be inputted verses -39576
and when B1 is greater then M/H column
I need the return to be "Past due "number of days between the dates"

Hope this makes sense.

Please help

Elena