I am accepting a date in one field

and number of days in other

i want to calculate due date and display in other field
say duedate

ALso
i want to derive and display
4 dates based on duedate
for example
if due date is 10-01-2009
the rest 4 dates should be( period of 3 months)
1. 09-04-2009
2. 09-07-2009
3. 09-10-2009
4. 09-01-2010

These dates to be displayed in fields
sdt1
sdt2
sdt3
sdt4
Please help

Simple date calculation;

var today = new Date();
var plus3 = new Date();
var plus12 = new Date();

plus3.setMonth(today.getMonth() + 3);
plus12.setMonth(today.getMonth() + 12);

to display the value;

document.formname.fieldname.value = dateVariable.toDateString();