When you are dismissing a Participant, there is a setting that dictates how the program end date is populated. The program end date can either be blank or have today's date by default. To change this setting:
Under Site Administration in the Navigation Bar, go to "Manage Add Participant/Family."
Scroll Down to the Dismiss Participants Configuration section
If you would like the program end date to be blank by default, make sure the "Make Program End Date (dismissal date) Blank by default" option is checked. If you would like the program end date to automatically populate with the current date, make sure this option is not checked.