Add Days to Calendar

Add Days to Calendar

I am creating an Invoice system.  I have a field called - Terms, which is a drop-down with four options since I have customers that pay on different lead times.
  1. Due on Receipt
  2. Net 30
  3. Net 45
  4. Net 60
I have another field that is a date field called - Due_Date.

Is there a way that when I enter a value from the Term drop-down, it will automatically fill in the Due_Date?

So If I enter a net 30 on August 15, then it would automatically fill in the Due_Date with a date 30 days from now?