You can change the time that your tickets go on sale, and the time your ticket sales end by logging in to your account, clicking on your event, and clicking on the Edit Event link from the left navigation menu. Scroll down to the Tickets section, you will see that there is a time you can select for tickets to Start Selling and to Stop Selling.

 

Ticket sales will automatically end once your event begins, so if your event can still sell tickets once it has commenced, you may need to alter the time your tickets sales end by selecting Specific Time from the drop down menu.