How do you calculate a date on a form?

I am working on a form for our customers and our staff to use and I needfor it to calculate a date from another date. We are currently using an Excel spreadsheet that is working very well, but we are starting to run into customers who do not have access to Excel. Having the form in Adobe would give us more flexibility.

Here is an example of exactly what I need:

Last Read Date: 01/01/00

Approximate Due Date_1: 01/26/00  (this is the Last Read Date + 26 days)

Approximate Due Date_2: 02/26/00  (this is the Last Read Date + 56 days)

Approximate Due Date_3: 03/26/00  (this is the Last Read Date + 85 days)

Thank you