From the iPhone manual:
When Time Zone Support is on, Calendar displays event dates and times in the time
zone of the city you selected. When Time Zone Support is off, Calendar displays events
in the time zone of your current location as determined by the network time.
Set iPhone to adjust event times for a selected time zone:
1 In Settings, choose “Mail, Contacts, Calendars.”
2 Under Calendars, tap Time Zone Support, then turn Time Zone Support on.
3 Tap Time Zone and search for a major city in the time zone you want.