The field type can be changed Edit>Fields.
Make sure the date field in the list and the new date field are both Date "type" fields.
You can add the new field to the list by clicking insert at the bottom of the Edit>Fields screen.
The new field name can be a field name of your preference. Change the (newdatefield) in the command to reflect the name of the field you added.
After you have the new field added. Go to Edit>Command and paste the following into the Command Statement Syntax Area.
Code: Select all
replace all newdatefield with date+15
The field name (newdatefield) will be the field you added in Edit>Fields for the newly calculated date to be saved in.
Example: An old date field with 06/01/2015 will become 06/16/2015 with this 15 day calculation command.