Simple, just navigate to the Dashboard menu for your event and select Event Details > Edit Dates. From here you can add a new date to the event by clicking the Add date button and filling out the form.
You can only add new dates into the system i.e. you can't have two dates on the 21st March 2017 for the same event. If you have different entry times for your event then add new tickets with different entry times. Details of adding new tickets can be found here.