Calculate number of days between invoice date and today's 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.

I hope I explained this correctly.

I think this might be a better formula for E4:

=if(B4<>"",if(B4>C4,"Overdue",""),"")

Also for D4, use conditional formatting: If the value >=0, then text = black, if the value <0, then the text = red.

take a look at this one:
Time Formula_2.xlsx

sorry wrong sheet

is it some thing like is one:
Help with Excel (1).xlsx

Or perhaps

Help with Excel_shavar.xlsx

Keep in mind the TODAY function counts the same day as received. I have similar situation but I use WORKDAYS with a list of corporate holidays to account for non-holiday business days. So that is another option.

Some thing like this Help with Excel.xlsx

This is exactly what I was looking for! Thank you!

How to prepare bill format in Excel ?

Hi Every one.............. i m biggner and very enthu to learn new formulas.

wrong calculation for 11 Feb cell