Microsoft Access DB Dates

Ok, i am setting up a Microsoft Access Database in which i have a table that has a Name field (the primary key), a Start Date field, and an End Date field. I created a form that has all the three fields and a button that inserts the data into my table when i am done. Basically i take the start date and add 21 days to it. What i want access to do, is automatically advance the date by 21, (like =[date]+21) and have it insert that into the db when i am done. I dont care if i see this on the form. I haven’t figure out how to do this. If someone here can simply tell me how to make it calculate the date and then insert that data into the db when i hit the button, i would be very happy. Thanks for any help you could provide.

-Matt Keller