Calculate Number Of Days Between Invoice And Today Date

I'm trying to create a new column which calculates the number of days from the today's date to the due date of an invoice. Also a Column that an invoice is/was overdue.

Column A - Clerk inputs date that invoice was received
Column B - Clerk inputs the date that the bill was paid
Column C - Auto populates the due date of the invoice (20 days after the invoice is received)
Column D - I want a number of days to auto populate based on today's date that will show how many days we have to pay the bill. Example: today is 2/19/2014, bill is due 2/22/2014 (Column C), column D should read 3 (I would prefer is the number is black for "we have days left to pay", red for "we're behind")
*Extra bonus for Column D, if the column goes blank after a date is entered into Column B* - but not necessary

Column E - I would like if the date the bill was paid (Column B) is greater than the date the invoice is due (Column C) to show "Overdue" in the cell.