Here is the code I use.
Use this code on the a text field.
Put the following code as Calculate in FormCalc(exclude the ------- divider):
----------beginning of the code-----------
if
(HasValue(xyz)) then
Num2Date( (Date2Num(xyz.formattedValue
, "MM/DD/YYYY") + ###), "MM/DD/YYYY")
else
null
endif
--------ending of the code--------
=> Replace xyz with the field name, which your employee fills in
=> Replace ### with the number of days you want to add to