Adding a time slot to the event is only possible for events created as advanced event type. How to create an event with time slots can be found here.
Click on EDIT DAYS in the upper part of the calendar
Click in the calendar on the day you want to adjust
Click on the plus (+) 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.