These are the timezones available under Plan > Event Setup > Details > Timezone:

Please note that GMT will be updated before and after daylight saving, where applicable.

International Date Line West
Midway Island, Samoa
Hawaii
Alaska
Pacific Time (US & Canada)
Tijuana, Baja California
Mountain Time (US & Canada)
Chihuahua, La Paz, Mazatlan - New
Chihuahua, La Paz, Mazatlan - Old
Arizona (GMT-07:00)
Guadalajara, Mexico City, Monterrey - Old
Saskatchewan
Central America
Central Time (US & Canada)
Guadalajara, Mexico City, Monterrey - New
Eastern Time (US & Canada)
Bogota, Lima, Quito, Rio Branco
Indiana (East)
Caracas
La Paz
Manaus
Atlantic Time (Canada)
Newfoundland
Montevideo
Santiago
Buenos Aires, Georgetown
Greenland
Mid-Atlantic
Brasilia
Azores
Cape Verde Is.
Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London
Monrovia, Reykjavik
Casablanca
West Central Africa
Amsterdam, Berlin, Bern, Rome, Stockholm, Vienna
Windhoek
Brussels, Copenhagen, Madrid, Paris
Belgrade, Bratislava, Budapest, Ljubljana, Prague
Sarajevo, Skopje, Warsaw, Zagreb
Helsinki, Kyiv, Riga, Sofia, Tallinn, Vilnius
Cairo
Harare, Pretoria
Beirut
Athens, Bucharest
Jerusalem
Amman
Moscow, St. Petersburg, Volgograd
Minsk
Nairobi
Kuwait, Riyadh
Baghdad
Istanbul
Tehran
Yerevan
Abu Dhabi, Muscat
Baku
Caucasus Standard Time
Tbilisi
Kabul
Ekaterinburg
Islamabad, Karachi, Tashkent
Chennai, Kolkata, Mumbai, New Delhi
Sri Jayawardenapura
Kathmandu
Astana, Dhaka
Almaty
Yangon (Rangoon)
Novosibirsk
Krasnoyarsk
Bangkok, Hanoi, Jakarta
Kuala Lumpur, Singapore
Perth
Irkutsk, Ulaan Bataar
Taipei
Beijing, Chongqing, Hong Kong, Urumqi
Seoul
Osaka, Sapporo, Tokyo
Yakutsk
Darwin
Adelaide
Canberra, Melbourne, Sydney
Brisbane
Hobart
Guam, Port Moresby
Vladivostok
Magadan, Solomon Is., New Caledonia
Fiji, Kamchatka, Marshall Is.
Auckland, Wellington
Nuku'alofa