Countdown from date but not to a date

Goal: to countdown from a start date without an end date. Just by adding due dates.
For Example: I have certain tasks that are due at different days from a start date. I want to be able to enter one start date and next to the task enter how many days it will take from that start date and then a counter to tell me how many days left.

Date: 5/14/2012

Task: Spreadsheet
Days to Complete: 15
Days Left Until Due: 13


I don't want to enter a date in the Days to Complete. My tasks are defaulted and standard for each workbook. Is there a way to do this?