When you create a new event, by default it’s assumed to be in whatever timezone you have set as the default for your website. However, you can change the timezone to a different one (specified by city). In the example below, the site defaults to U.S. Eastern time, but I can change that by specifying Los Angeles time.
The checkboxes on that new event screen specify that the timezone should be displayed and that the option to convert to the user’s local timezone should be offered. That causes the Show in My Timezone button to be displayed, as shown below. Click on the button to see the translated time (based on the local timezone reported by your computer).
Changing the Timezone on an Existing Event
If you didn’t set the timezone initially, you can change it on the RSVP / Event Options screen. (As of this writing, you can’t change it from the sidebar within the editor.)