Forms :: Access 2010 Auto Update Calculated Date Field?

I have a database to track temporary decertification's. I have the expiration and max dates calculated out from the original dates at the top of each box. The temp expiration date is calculated by adding 267 days from the first date . When we enter an extension, the new expiration date is 30 days from the extension date. My question is, how can I make the expiration date update when a new extension is put in.

For ex.

Temp Decert Date: 05 Dec 2014
Temp Decert Extens 1:
Temp Decert Extens 2:
Temp Decert Extens 3:
Temp Experation Date: 31 Aug 2015
Max Temp Date: 04 Dec 2015

how can I make the expiration date update to go 30 days from what is in the extens field 1, 2, and 3 (respectively) instead of 267 days from the original date?

So I want it to look like this after updating a field

Temp Decert Date: 05 Dec 2014
Temp Decert Extens 1: 30 Aug 2015
Temp Decert Extens 2:
Temp Decert Extens 3:
Temp Experation Date: 29 Sep 2015
Max Temp Date: 04 Dec 2015