date plus number of days equals day of the week and date

Hi,
I would like to know how to create the code for the example below.
Pick a date on the calendar then in another box enter number of days then hit calculate which will give day of the week with date.

For example august 5, 2013 plus 10 days = Thursday August 15, 2013.

Thanks

http://wordpress.org/plugins/calculated-fields-form/