Date calculations

[Dottor Zivago]

Dear friends,
Sorry for another basic question to the list but:
I would like to have a field as a container for a date, then a button that
performs a calculation on that date and puts the result in another field.
For instance, for a given LMP (last menstrual period) I need to calculate an
EDD (estimated date of delivery) that is simply LMP+280 (and this is what I
do in Excel or FileMaker).
Pleeeease, assume an IQ<=40; some lines to cut and paste between "on
mouseup" and "end mouseup" would be very very appreciated.
Thank you very much
Ciao
Ubaldo