Retrieve local date and time of an arbitrary day in an arbitrary timezone, considering Daylight Saving Time

This function does not exist native in javascript, it has to be done "by hand" or else using a library.

So here's one I did now (between last night and today):

First, declare all time zones in a JSON:

var olson = '{"result":[{"TimeZoneId":"Africa/Abidjan","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Accra","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Addis Ababa","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Algiers","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Asmara","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Bamako","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Bangui","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Banjul","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Bissau","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Blantyre","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Brazzaville","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Bujumbura","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Cairo","DST":"3","GMT":"2"},{"TimeZoneId":"Africa/Casablanca","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Ceuta","DST":"2","GMT":"1"},{"TimeZoneId":"Africa/Conakry","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Dakar","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Dar es Salaam","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Djibouti","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Douala","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/El Aaiun","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Freetown","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Gaborone","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Harare","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Johannesburg","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Kampala","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Khartoum","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Kigali","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Kinshasa","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Lagos","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Libreville","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Lome","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Luanda","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Lubumbashi","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Lusaka","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Malabo","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Maputo","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Maseru","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Mbabane","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Mogadishu","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Monrovia","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Nairobi","DST":"3","GMT":"3"},{"TimeZoneId":"Africa/Ndjamena","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Niamey","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Nouakchott","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Ouagadougou","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Porto/Novo","DST":"1","GMT":"1"},{"TimeZoneId":"Africa/Sao Tome","DST":"0","GMT":"0"},{"TimeZoneId":"Africa/Tripoli","DST":"2","GMT":"2"},{"TimeZoneId":"Africa/Tunis","DST":"2","GMT":"1"},{"TimeZoneId":"Africa/Windhoek","DST":"1","GMT":"2"},{"TimeZoneId":"America/Adak","DST":"-9","GMT":"-10"},{"TimeZoneId":"America/Anchorage","DST":"-8","GMT":"-9"},{"TimeZoneId":"America/Anguilla","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Antigua","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Araguaina","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Buenos Aires","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Catamarca","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Cordoba","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Jujuy","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/La Rioja","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Mendoza","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Rio Gallegos","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Salta","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/San Juan","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/San Luis","DST":"-4","GMT":"-3"},{"TimeZoneId":"America/Argentina/Tucuman","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Argentina/Ushuaia","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Aruba","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Asuncion","DST":"-4","GMT":"-3"},{"TimeZoneId":"America/Atikokan","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Bahia","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Barbados","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Belem","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Belize","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Blanc/Sablon","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Boa Vista","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Bogota","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Boise","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Cambridge Bay","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Campo Grande","DST":"-4","GMT":"-3"},{"TimeZoneId":"America/Cancun","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Caracas","DST":"-4.5","GMT":"-4.5"},{"TimeZoneId":"America/Cayenne","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Cayman","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Chicago","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Chihuahua","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Costa Rica","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Cuiaba","DST":"-4","GMT":"-3"},{"TimeZoneId":"America/Curacao","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Danmarkshavn","DST":"0","GMT":"0"},{"TimeZoneId":"America/Dawson","DST":"-7","GMT":"-8"},{"TimeZoneId":"America/Dawson Creek","DST":"-7","GMT":"-7"},{"TimeZoneId":"America/Denver","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Detroit","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Dominica","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Edmonton","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Eirunepe","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/El Salvador","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Fortaleza","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Glace Bay","DST":"-3","GMT":"-4"},{"TimeZoneId":"America/Godthab","DST":"-2","GMT":"-3"},{"TimeZoneId":"America/Goose Bay","DST":"-3","GMT":"-4"},{"TimeZoneId":"America/Grand Turk","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Grenada","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Guadeloupe","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Guatemala","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Guayaquil","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Guyana","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Halifax","DST":"-3","GMT":"-4"},{"TimeZoneId":"America/Havana","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Hermosillo","DST":"-7","GMT":"-7"},{"TimeZoneId":"America/Indiana/Indianapolis","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Indiana/Knox","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Indiana/Marengo","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Indiana/Petersburg","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Indiana/Tell City","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Indiana/Vevay","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Indiana/Vincennes","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Indiana/Winamac","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Inuvik","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Iqaluit","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Jamaica","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Juneau","DST":"-8","GMT":"-9"},{"TimeZoneId":"America/Kentucky/Louisville","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Kentucky/Monticello","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/La Paz","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Lima","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Los Angeles","DST":"-7","GMT":"-8"},{"TimeZoneId":"America/Maceio","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Managua","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Manaus","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Marigot","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Martinique","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Matamoros","DST":"0","GMT":"0"},{"TimeZoneId":"America/Mazatlan","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/Menominee","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Merida","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Mexico City","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Miquelon","DST":"-2","GMT":"-3"},{"TimeZoneId":"America/Moncton","DST":"-3","GMT":"-4"},{"TimeZoneId":"America/Monterrey","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Montevideo","DST":"-3","GMT":"-2"},{"TimeZoneId":"America/Montreal","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Montserrat","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Nassau","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/New York","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Nipigon","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Nome","DST":"-8","GMT":"-9"},{"TimeZoneId":"America/Noronha","DST":"-2","GMT":"-2"},{"TimeZoneId":"America/North Dakota/Center","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/North Dakota/New Salem","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Ojinaga","DST":"0","GMT":"0"},{"TimeZoneId":"America/Panama","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Pangnirtung","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Paramaribo","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Phoenix","DST":"-7","GMT":"-7"},{"TimeZoneId":"America/Port/au/Prince","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Port of Spain","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Porto Velho","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Puerto Rico","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Rainy River","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Rankin Inlet","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Recife","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Regina","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Resolute","DST":"-5","GMT":"-5"},{"TimeZoneId":"America/Rio Branco","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Santa Isabel","DST":"0","GMT":"0"},{"TimeZoneId":"America/Santarem","DST":"-3","GMT":"-3"},{"TimeZoneId":"America/Santiago","DST":"-4","GMT":"-3"},{"TimeZoneId":"America/Santo Domingo","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Sao Paulo","DST":"-3","GMT":"-2"},{"TimeZoneId":"America/Scoresbysund","DST":"0","GMT":"-1"},{"TimeZoneId":"America/Shiprock","DST":"-6","GMT":"-7"},{"TimeZoneId":"America/St Barthelemy","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/St Johns","DST":"-2.5","GMT":"-3.5"},{"TimeZoneId":"America/St Kitts","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/St Lucia","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/St Thomas","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/St Vincent","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Swift Current","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Tegucigalpa","DST":"-6","GMT":"-6"},{"TimeZoneId":"America/Thule","DST":"-3","GMT":"-4"},{"TimeZoneId":"America/Thunder Bay","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Tijuana","DST":"-7","GMT":"-8"},{"TimeZoneId":"America/Toronto","DST":"-4","GMT":"-5"},{"TimeZoneId":"America/Tortola","DST":"-4","GMT":"-4"},{"TimeZoneId":"America/Vancouver","DST":"-7","GMT":"-8"},{"TimeZoneId":"America/Whitehorse","DST":"-7","GMT":"-8"},{"TimeZoneId":"America/Winnipeg","DST":"-5","GMT":"-6"},{"TimeZoneId":"America/Yakutat","DST":"-8","GMT":"-9"},{"TimeZoneId":"America/Yellowknife","DST":"-6","GMT":"-7"},{"TimeZoneId":"Antarctica/Casey","DST":"11","GMT":"11"},{"TimeZoneId":"Antarctica/Davis","DST":"5","GMT":"5"},{"TimeZoneId":"Antarctica/DumontDUrville","DST":"10","GMT":"10"},{"TimeZoneId":"Antarctica/Mawson","DST":"5","GMT":"5"},{"TimeZoneId":"Antarctica/McMurdo","DST":"12","GMT":"13"},{"TimeZoneId":"Antarctica/Palmer","DST":"-4","GMT":"-3"},{"TimeZoneId":"Antarctica/Rothera","DST":"-3","GMT":"-3"},{"TimeZoneId":"Antarctica/South Pole","DST":"12","GMT":"13"},{"TimeZoneId":"Antarctica/Syowa","DST":"3","GMT":"3"},{"TimeZoneId":"Antarctica/Vostok","DST":"6","GMT":"6"},{"TimeZoneId":"Arctic/Longyearbyen","DST":"2","GMT":"1"},{"TimeZoneId":"Asia/Aden","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Almaty","DST":"6","GMT":"6"},{"TimeZoneId":"Asia/Amman","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Anadyr","DST":"13","GMT":"12"},{"TimeZoneId":"Asia/Aqtau","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Aqtobe","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Ashgabat","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Baghdad","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Bahrain","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Baku","DST":"5","GMT":"4"},{"TimeZoneId":"Asia/Bangkok","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Beirut","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Bishkek","DST":"6","GMT":"6"},{"TimeZoneId":"Asia/Brunei","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Choibalsan","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Chongqing","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Colombo","DST":"5.5","GMT":"5.5"},{"TimeZoneId":"Asia/Damascus","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Dhaka","DST":"7","GMT":"6"},{"TimeZoneId":"Asia/Dili","DST":"9","GMT":"9"},{"TimeZoneId":"Asia/Dubai","DST":"4","GMT":"4"},{"TimeZoneId":"Asia/Dushanbe","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Gaza","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Harbin","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Ho Chi Minh","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Hong Kong","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Hovd","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Irkutsk","DST":"9","GMT":"8"},{"TimeZoneId":"Asia/Jakarta","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Jayapura","DST":"9","GMT":"9"},{"TimeZoneId":"Asia/Jerusalem","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Kabul","DST":"4.5","GMT":"4.5"},{"TimeZoneId":"Asia/Kamchatka","DST":"13","GMT":"12"},{"TimeZoneId":"Asia/Karachi","DST":"6","GMT":"5"},{"TimeZoneId":"Asia/Kashgar","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Kathmandu","DST":"5.75","GMT":"5.75"},{"TimeZoneId":"Asia/Kolkata","DST":"5.5","GMT":"5.5"},{"TimeZoneId":"Asia/Krasnoyarsk","DST":"8","GMT":"7"},{"TimeZoneId":"Asia/Kuala Lumpur","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Kuching","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Kuwait","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Macau","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Magadan","DST":"12","GMT":"11"},{"TimeZoneId":"Asia/Makassar","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Manila","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Muscat","DST":"4","GMT":"4"},{"TimeZoneId":"Asia/Nicosia","DST":"3","GMT":"2"},{"TimeZoneId":"Asia/Novokuznetsk","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Novosibirsk","DST":"7","GMT":"6"},{"TimeZoneId":"Asia/Omsk","DST":"7","GMT":"6"},{"TimeZoneId":"Asia/Oral","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Phnom Penh","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Pontianak","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Pyongyang","DST":"9","GMT":"9"},{"TimeZoneId":"Asia/Qatar","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Qyzylorda","DST":"6","GMT":"6"},{"TimeZoneId":"Asia/Rangoon","DST":"6.5","GMT":"6.5"},{"TimeZoneId":"Asia/Riyadh","DST":"3","GMT":"3"},{"TimeZoneId":"Asia/Sakhalin","DST":"11","GMT":"10"},{"TimeZoneId":"Asia/Samarkand","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Seoul","DST":"9","GMT":"9"},{"TimeZoneId":"Asia/Shanghai","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Singapore","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Taipei","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Tashkent","DST":"5","GMT":"5"},{"TimeZoneId":"Asia/Tbilisi","DST":"4","GMT":"4"},{"TimeZoneId":"Asia/Tehran","DST":"4.5","GMT":"3.5"},{"TimeZoneId":"Asia/Thimphu","DST":"6","GMT":"6"},{"TimeZoneId":"Asia/Tokyo","DST":"9","GMT":"9"},{"TimeZoneId":"Asia/Ulaanbaatar","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Urumqi","DST":"8","GMT":"8"},{"TimeZoneId":"Asia/Vientiane","DST":"7","GMT":"7"},{"TimeZoneId":"Asia/Vladivostok","DST":"11","GMT":"10"},{"TimeZoneId":"Asia/Yakutsk","DST":"10","GMT":"9"},{"TimeZoneId":"Asia/Yekaterinburg","DST":"6","GMT":"5"},{"TimeZoneId":"Asia/Yerevan","DST":"5","GMT":"4"},{"TimeZoneId":"Atlantic/Azores","DST":"0","GMT":"-1"},{"TimeZoneId":"Atlantic/Bermuda","DST":"-3","GMT":"-4"},{"TimeZoneId":"Atlantic/Canary","DST":"1","GMT":"0"},{"TimeZoneId":"Atlantic/Cape Verde","DST":"-1","GMT":"-1"},{"TimeZoneId":"Atlantic/Faroe","DST":"1","GMT":"0"},{"TimeZoneId":"Atlantic/Madeira","DST":"1","GMT":"0"},{"TimeZoneId":"Atlantic/Reykjavik","DST":"0","GMT":"0"},{"TimeZoneId":"Atlantic/South Georgia","DST":"-2","GMT":"-2"},{"TimeZoneId":"Atlantic/St Helena","DST":"0","GMT":"0"},{"TimeZoneId":"Atlantic/Stanley","DST":"-4","GMT":"-3"},{"TimeZoneId":"Australia/Adelaide","DST":"9.5","GMT":"10.5"},{"TimeZoneId":"Australia/Brisbane","DST":"10","GMT":"10"},{"TimeZoneId":"Australia/Broken Hill","DST":"9.5","GMT":"10.5"},{"TimeZoneId":"Australia/Currie","DST":"10","GMT":"11"},{"TimeZoneId":"Australia/Darwin","DST":"9.5","GMT":"9.5"},{"TimeZoneId":"Australia/Eucla","DST":"8.75","GMT":"8.75"},{"TimeZoneId":"Australia/Hobart","DST":"10","GMT":"11"},{"TimeZoneId":"Australia/Lindeman","DST":"10","GMT":"10"},{"TimeZoneId":"Australia/Lord Howe","DST":"10.5","GMT":"11"},{"TimeZoneId":"Australia/Melbourne","DST":"10","GMT":"11"},{"TimeZoneId":"Australia/Perth","DST":"8","GMT":"8"},{"TimeZoneId":"Australia/Sydney","DST":"10","GMT":"11"},{"TimeZoneId":"Europe/Amsterdam","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Andorra","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Athens","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Belgrade","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Berlin","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Bratislava","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Brussels","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Bucharest","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Budapest","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Chisinau","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Copenhagen","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Dublin","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Gibraltar","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Guernsey","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Helsinki","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Isle of Man","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Istanbul","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Jersey","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Kaliningrad","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Kiev","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Lisbon","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Ljubljana","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/London","DST":"1","GMT":"0"},{"TimeZoneId":"Europe/Luxembourg","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Madrid","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Malta","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Mariehamn","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Minsk","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Monaco","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Moscow","DST":"4","GMT":"3"},{"TimeZoneId":"Europe/Oslo","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Paris","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Podgorica","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Prague","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Riga","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Rome","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Samara","DST":"5","GMT":"4"},{"TimeZoneId":"Europe/San Marino","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Sarajevo","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Simferopol","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Skopje","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Sofia","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Stockholm","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Tallinn","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Tirane","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Uzhgorod","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Vaduz","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Vatican","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Vienna","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Vilnius","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Volgograd","DST":"4","GMT":"3"},{"TimeZoneId":"Europe/Warsaw","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Zagreb","DST":"2","GMT":"1"},{"TimeZoneId":"Europe/Zaporozhye","DST":"3","GMT":"2"},{"TimeZoneId":"Europe/Zurich","DST":"2","GMT":"1"},{"TimeZoneId":"Indian/Antananarivo","DST":"3","GMT":"3"},{"TimeZoneId":"Indian/Chagos","DST":"6","GMT":"6"},{"TimeZoneId":"Indian/Christmas","DST":"7","GMT":"7"},{"TimeZoneId":"Indian/Cocos","DST":"6.5","GMT":"6.5"},{"TimeZoneId":"Indian/Comoro","DST":"3","GMT":"3"},{"TimeZoneId":"Indian/Kerguelen","DST":"5","GMT":"5"},{"TimeZoneId":"Indian/Mahe","DST":"4","GMT":"4"},{"TimeZoneId":"Indian/Maldives","DST":"5","GMT":"5"},{"TimeZoneId":"Indian/Mauritius","DST":"4","GMT":"4"},{"TimeZoneId":"Indian/Mayotte","DST":"3","GMT":"3"},{"TimeZoneId":"Indian/Reunion","DST":"4","GMT":"4"},{"TimeZoneId":"Pacific/Apia","DST":"-11","GMT":"-10"},{"TimeZoneId":"Pacific/Auckland","DST":"12","GMT":"13"},{"TimeZoneId":"Pacific/Chatham","DST":"12.75","GMT":"13.75"},{"TimeZoneId":"Pacific/Easter","DST":"-6","GMT":"-5"},{"TimeZoneId":"Pacific/Efate","DST":"11","GMT":"11"},{"TimeZoneId":"Pacific/Enderbury","DST":"13","GMT":"13"},{"TimeZoneId":"Pacific/Fakaofo","DST":"-10","GMT":"-10"},{"TimeZoneId":"Pacific/Fiji","DST":"12","GMT":"13"},{"TimeZoneId":"Pacific/Funafuti","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Galapagos","DST":"-6","GMT":"-6"},{"TimeZoneId":"Pacific/Gambier","DST":"-9","GMT":"-9"},{"TimeZoneId":"Pacific/Guadalcanal","DST":"11","GMT":"11"},{"TimeZoneId":"Pacific/Guam","DST":"10","GMT":"10"},{"TimeZoneId":"Pacific/Honolulu","DST":"-10","GMT":"-10"},{"TimeZoneId":"Pacific/Johnston","DST":"-10","GMT":"-10"},{"TimeZoneId":"Pacific/Kiritimati","DST":"14","GMT":"14"},{"TimeZoneId":"Pacific/Kosrae","DST":"11","GMT":"11"},{"TimeZoneId":"Pacific/Kwajalein","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Majuro","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Marquesas","DST":"-9.5","GMT":"-9.5"},{"TimeZoneId":"Pacific/Midway","DST":"-11","GMT":"-11"},{"TimeZoneId":"Pacific/Nauru","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Niue","DST":"-11","GMT":"-11"},{"TimeZoneId":"Pacific/Norfolk","DST":"11.5","GMT":"11.5"},{"TimeZoneId":"Pacific/Noumea","DST":"11","GMT":"11"},{"TimeZoneId":"Pacific/Pago Pago","DST":"-11","GMT":"-11"},{"TimeZoneId":"Pacific/Palau","DST":"9","GMT":"9"},{"TimeZoneId":"Pacific/Pitcairn","DST":"-8","GMT":"-8"},{"TimeZoneId":"Pacific/Ponape","DST":"11","GMT":"11"},{"TimeZoneId":"Pacific/Port Moresby","DST":"10","GMT":"10"},{"TimeZoneId":"Pacific/Rarotonga","DST":"-10","GMT":"-10"},{"TimeZoneId":"Pacific/Saipan","DST":"10","GMT":"10"},{"TimeZoneId":"Pacific/Tahiti","DST":"-10","GMT":"-10"},{"TimeZoneId":"Pacific/Tarawa","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Tongatapu","DST":"13","GMT":"13"},{"TimeZoneId":"Pacific/Truk","DST":"10","GMT":"10"},{"TimeZoneId":"Pacific/Wake","DST":"12","GMT":"12"},{"TimeZoneId":"Pacific/Wallis","DST":"12","GMT":"12"}]}';
olson = JSON.parse(olson);
var olsonKeys = Object.keys(olson);

Having an as timezone and its time Offset, you can use this function:

function gerarData(a, b) {
    var match = {};
    for (var i = 0; i < olson.result.length; i++) {
        if (olson.result[i].TimeZoneId == b) match = olson.result[i];
    }

    var clientOffset = new Date().getTimezoneOffset() / 60;
    var timeZoneDSTOffset = parseInt(match.GMT);
    var offsetCorrection = (timeZoneDSTOffset - 3) * 60 * 60 * 1000; // *1000 porque o javascript trabalha em milisegundos
    var date = new Date(a * 1000 + offsetCorrection); 

    // função para corrigir numero de digitos para ter sempre 2
    function digitos(n) {
        return n > 9 ? "" + n : "0" + n;
    }
    // formatar a data
    var formatedDate = date.getFullYear() + '/' + 
        digitos(date.getMonth() + 1) + '/' +  // +1 porque o getMonth() começa em zero
        digitos(date.getDate()) + ' ' + 
        digitos(date.getHours()) + ':' + 
        digitos(date.getMinutes()) + ':' + 
        digitos(date.getSeconds());

    return formatedDate;
}

Demo

So in my demo:

gerarData(1357041600, "Europe/London");     // dá "2013/01/01 10:00:00"
gerarData(1372680000, "America/Sao Paulo"); // dá "2013/07/01 09:00:00"

Note that this my suggestion does not accept ã and others characters, but this you can fix with this question / answer.

This result is much simpler in PHP, you could create an ajax request and go get this value, but if I understood correctly this is what you do not want.

With native JavaScript, the only way is to use some solution similar to what is suggested in Sergio's answer .

But if you can use a library, I suggest the Moment.js . To work with time zones, you also need to have the Moment Timezone.

An advantage of using this library is that time zone information changes all the time and it is difficult to keep your code abreast of all the rules (for example, in Brazil a Daylight Saving Time rule recently changed ). in the documentation explains how to keep the data up to date.

With this library, the code is very easy:

function f(t, s) {
    return moment.tz(t * 1000, s).format('YYYY/MM/DD HH:mm:ss');
}

I had to multiply t by 1000 because the values you indicated are in seconds, but the API works with milliseconds. To use:

function f(timestamp, timezone) {
    return moment.tz(timestamp * 1000, timezone).format('YYYY/MM/DD HH:mm:ss');
}

console.log(f(1357041600, "America/Sao_Paulo")); // 2013/01/01 10:00:00
console.log(f(1372680000, "America/Sao_Paulo")); // 2013/07/01 09:00:00

<script src="https://momentjs.com/downloads/moment.min.js"></script>
<script src="https://momentjs.com/downloads/moment-timezone-with-data.min.js"></script>

Note that the second parameter is America/Sao_Paulo: without accentuation and with a _ instead of space. That's because the names of timezones follow the IANA standard .

If you want, you can adapt the code to accept more names, but always mapping to a valid name. To know all the available names, use the function moment.tz.names(), which returns an array with all the names. Ex:

function f(timestamp, timezone) {
    // se o timezone não existe, usa algum valor default
    if (! moment.tz.names().includes(timezone)) {
        timezone = 'America/Sao_Paulo';
    }
    return moment.tz(timestamp * 1000, timezone).format('YYYY/MM/DD HH:mm:ss');
}

console.log(f(1357041600, "America/Sao_Paulo")); // 2013/01/01 10:00:00
console.log(f(1372680000, "timezone invalido")); // 2013/07/01 09:00:00

<script src="https://momentjs.com/downloads/moment.min.js"></script>
<script src="https://momentjs.com/downloads/moment-timezone-with-data.min.js"></script>

You would have to periodically download the historical list of timezones at http://www.iana.org/time-zones, on your server, interpret this document to a more palatable format in Javascript (JSON?) and load this document at the time of performing its function "f", so that it can search the timezone by name.

Java seems to have this information too, directly via API, so another way out would be to load a Java applet that your Javascript code then you could consult. This is if you can force clients to use Java in the browser and the best solution (server-side solving) is not feasible.

Note that your function " f " does not need to communicate with the server at every call. Just once to load the table, and if the table is converted to JSON, it can be loaded as an extra js source on the page.