iOS App + Web Portal:
You can specify a time slot for an Artist you are interested in booking during the request process. When you hit Send Request on the profile of an artist, you will then be asked to designate an event for the artist to play. When you select the event you're interested in booking the artist for, you will be able to specify which time slot you want to the DJ to fill using the input boxes under Time Slot. Set the start and end times for the artist before proceeding to send the booking request.