Hi there,
to do this you I suggest you to use the language FormCalc, it can use functions which are easy to use to manipulate dates
Date2Num(date, format) (returns the number of days since 1st January 1900)
and
Num2Date(number, format) (returns a date format)
The date field which you want it to be readOnly will need to have the following FormCalc script within the calculate event
Hope this help!