Guys...Can anyone help ??

I have a client who wants to have the Milestone "date" shown as "Day x" where Day x is the cumulative amount of days from the Project start date eg if the project was to start on 1st January and the first muilestone was on 31st January, they want the Milestone to display "Day 31". I have had a play with trying to put a formula into the Start1 field but alas no joy.

Any ideas how I can achieve this ??