Calculator Compute Automatically After Inputting The Date

i m making a very small calculator. It's a little hard to describe, but I need to look like the following
Let's say the individual got here on 1 Jan

Phase 1 is for days 1-14 last for 14 days
Phase 2 is for Day 15-35 last for 21 days
Phase 3 is day 36+

This is what I need it to look like: All the phase information will compute automatically after inputting the date arrived. In addition I would also like to be able to change one of the phase dates and the remainder of the phases compensate according to the newly entered date.

Date Arrived = 01 Jan 09
Phase 1 = 01jan09
Phase 2 = 04Feb09
Phase 3 = 05 Feb09