For a convenient way of displaying the running time of an Event, you can use a Time Difference Control. Time Difference fields automatically calculate the duration of an event by taking the difference between two fields - a start date and an end date. Whenever you change either date, the field will update automatically with the new time.
When you create a Time Difference control, it should be in the same "Container" as either 2 Date Fields or 2 Date Time Fields.
The fields should consist of one field with "Start" in its label and another with "End" in its label, so that the TimeDifference control knows which fields to calculate from.
You don't need to put anything into the Time Difference field - it does it automatically. Instead, put a date/time in the Start and End fields as you normally would. When you see a yellow flash on the Time Difference control field, it means it has updated successfully.
If you have two or more fields that both say "Start" or both say "End", the Time Difference field will calculate using the most recently made field.