The other thread has a link to the list of timezones. ISO8601 is just a standard for formatting timestamps. The timezones are covered by another standard for timezones called IANA.
With expressions, the timezone will automatically be pulled from the location associated with your account, so just make sure to set a timezone on your location.
formatDate($context.event.timestamp, "L LT")
Alternatively, if you want to explicitly override the timezone, you’re looking for America/New_York.
timezone = "America/New_York"
formatDate($context.event.timestamp, "L LT")
Update: With the release of Expressions, we would recommend using those directly as noted in the edited post above.