Each Shift must have a Begin and End Time - Shifts until Close
Every shift must have a start and end time. If your shifts do not have specific times, you can enter estimated or sample times and then make a notation in the Shift Description about the shift details.
For example, if the helper only helps until closing times which can vary - you can use the latest possible end time for the shift and then enter into the shift description "Help until Close" or something similar. Later you can adjust the end time to reflect the actual time helped if you like.