date calculation

It looks like you posted an empty question on this thread. Since you only titled your thread as "date calculation", I'll just list a few things that might help you get started.

1. The simplest way is to use this widget called Dates Difference http://widgets.jotform.com/widget/dates_difference This widget will allow you to add two calendar date pickers that will be used to get the difference between those two dates.

2. Another approach is to use the Form Calculation widget http://widgets.jotform.com/widget/calculation With this widget, you can perform simple to complex calculations, even with dates. Here's a very detailed guide to get you started How-to-Perform-Calculation-in-the-Form 

Nonetheless, if there's something specific that you want to achieve, then just give us more info so we can better assist you.