When setting up a new event, you have the option of making it a single-day (default) or multiple-day event.
The default setting is for single-day events. For this reason, the 'Start Date' field is editable, but the 'End Date' field is not. When you add your event's Start Date, Seatrobot automatically adds this same date as the event's End Date.
On your event's Register Page, the date format displays like this:
You can override this by clicking the 'Multiple Day Event' checkbox in Event Setup:
When checked, the End Day field becomes editable and you can add an End Date different from the Start Date, and save. Multiple day events' dates display like this on the event's Register Page: