Adding value to a date

I have 3 fields:

1. EndDate (Date field)
   
2. Adjust (will be a value of 1, 2, 3, 4, etc. or even negative -1-2,-3, etc.)
   
3. NewDate.
   I want to be able to display a new date in the EndDate field calcuated by adding the value entered in Adjust to the EndDate .
   Eg.
   
   EndDate + Adjust = NewDate
   
   13/02/2009 + 1 = 14/02/2009
   
   or
   
   18/02/2009 + -2 = 16/02/2009
   I want to use this to show delays in a completion date of a project.
   Thanks for help.