Adding a time slot to the event is only possible for events created as advanced event type.
Click on ADJUST DAYS above the calendar.
Click on the day you want to edit in the calendar
Click on the plus (+) sign to add a time slot
Enter the details and click on SAVE
After this you can use the plus (+) to add more time slots if necessary.
Please note: added time slots are immediately bookable for ticket buyers when the event is on sale.