To change the date and time on your event, go into Edit Mode by clicking on "Edit" on your event page. Then click on the date and time, and a calendar will open up.
At the bottom of the calendar view, you'll see an option to select and change your timezone.
Your guests won't be notified of any date or time changes - you'll need to send them a text blast if you'd like to notify them!
You can't change the date and time for events that are in the past.