When you create your event, the timezone will automatically default to whatever you chose when you created your box office. You can update your default timezone at any time by going to 'Box office settings' > 'Basic settings' > 'Timezone':

You can also update your timezone at the event level when you're creating the event. Under 'Event settings' there is a dropdown to choose which timezone you want to display on your event page:

Here's how it looks on the event page: