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JavaScript

var offset = (new Date()).getTimezoneOffset();


var timezones = {
    '-12': 'Pacific/Kwajalein',
    '-11': 'Pacific/Samoa',
    '-10': 'Pacific/Honolulu',
    '-9': 'America/Juneau',
    '-8': 'America/Los_Angeles',
    '-7': 'America/Denver',
    '-6': 'America/Mexico_City',
    '-5': 'America/New_York',
    '-4': 'America/Caracas',
    '-3.5': 'America/St_Johns',
    '-3': 'America/Argentina/Buenos_Aires',
    '-2': 'Atlantic/Azores',
    '-1': 'Atlantic/Azores',
    '0': 'Europe/London',
    '1': 'Europe/Paris',
    '2': 'Europe/Helsinki',
    '3': 'Europe/Moscow',
    '3.5': 'Asia/Tehran',
    '4': 'Asia/Baku',
    '4.5': 'Asia/Kabul',
    '5': 'Asia/Karachi',
    '5.5': 'Asia/Calcutta',
    '6': 'Asia/Colombo',
    '7': 'Asia/Bangkok',
    '8': 'Asia/Singapore',
    '9': 'Asia/Tokyo',
    '9.5': 'Australia/Darwin',
    '10': 'Pacific/Guam',
    '11': 'Asia/Magadan',
    '12': 'Asia/Kamchatka' 
};


alert(timezones[-offset / 60]);