tz.js

/* 
 * Original script by Josh Fraser (http://www.onlineaspect.com)
 * Continued and maintained by Jon Nylander at https://bitbucket.org/pellepim/jstimezonedetect
 *
 * Provided under the Do Whatever You Want With This Code License.
 */
/**
 * Namespace to hold all the code for timezone detection.
 */
var jstz = {};
jstz.HEMISPHERE_SOUTH = 'SOUTH';
jstz.HEMISPHERE_NORTH = 'NORTH';
jstz.HEMISPHERE_UNKNOWN = 'N/A';
jstz.olson = {};

/**
 * A simple object containing information of utc_offset, which olson timezone key to use, 
 * and if the timezone cares about daylight savings or not.
 * 
 * @constructor
 * @param {string} offset - for example '-11:00'
 * @param {string} olson_tz - the olson Identifier, such as "America/Denver"
 * @param {boolean} uses_dst - flag for whether the time zone somehow cares about daylight savings.
 */
jstz.TimeZone = function (offset, olson_tz, uses_dst) {
	this.utc_offset = offset;
	this.olson_tz = olson_tz;
	this.uses_dst = uses_dst;
};

/**
 * Prints out the result.
 * But before it does that, it calls this.ambiguity_check.
 */
jstz.TimeZone.prototype.display = function () {
	this.ambiguity_check();
	var response_text = '<b>UTC-offset</b>: ' + this.utc_offset + '<br/>';
	response_text += '<b>Zoneinfo key</b>: ' + this.olson_tz + '<br/>';
	response_text += '<b>Zone uses DST</b>: ' + (this.uses_dst ? 'yes' : 'no') + '<br/>';
	
	return response_text;
};

/**
 * Checks if a timezone has possible ambiguities. I.e timezones that are similar.
 * 
 * If the preliminary scan determines that we're in America/Denver. We double check
 * here that we're really there and not in America/Mazatlan.
 * 
 * This is done by checking known dates for when daylight savings start for different
 * timezones.
 */
jstz.TimeZone.prototype.ambiguity_check = function () {
	var ambiguity_list, length, i, tz;
	ambiguity_list = jstz.olson.ambiguity_list[this.olson_tz];
	
	if (typeof (ambiguity_list) === 'undefined') {
		return;
	}
	
	length = ambiguity_list.length;
	i = 0;
	
	for (; i < length; i += 1) {
		tz = ambiguity_list[i];

		if (jstz.date_is_dst(jstz.olson.dst_start_dates[tz])) {
			this.olson_tz = tz;
			return;
		}	
	}
};

/**
 * Checks whether a given date is in daylight savings time.
 * 
 * If the date supplied is after june, we assume that we're checking
 * for southern hemisphere DST.
 * 
 * @param {Date} date
 * @returns {boolean}
 */
jstz.date_is_dst = function (date) {
	var date_offset, base_offset; 
	base_offset = ((date.getMonth() > 5 ? jstz.get_june_offset() 
										: jstz.get_january_offset()));
	
	date_offset = jstz.get_date_offset(date);
	
	return (base_offset - date_offset) !== 0;
};

/** 
 * Gets the offset in minutes from UTC for a certain date.
 * 
 * @param date
 * @returns {number}
 */
jstz.get_date_offset = function (date) {
	return -date.getTimezoneOffset();
};

/**
 * This function does some basic calculations to create information about 
 * the user's timezone.
 * 
 * Returns a primitive object on the format
 * {'utc_offset' : -9, 'dst': 1, hemisphere' : 'north'}
 * where dst is 1 if the region uses daylight savings.
 * 
 * @returns {Object}  
 */
jstz.get_timezone_info = function () {
	var january_offset, june_offset, diff;
	january_offset = jstz.get_january_offset();
	june_offset = jstz.get_june_offset();
	diff = january_offset - june_offset;

	if (diff < 0) {
	    return {
            'utc_offset' : january_offset,
            'dst': 1,
            'hemisphere' : jstz.HEMISPHERE_NORTH
	    };
	} else if (diff > 0) {
        return {
            'utc_offset' : june_offset,
            'dst' : 1,
            'hemisphere' : jstz.HEMISPHERE_SOUTH
        };
	}

    return {
        'utc_offset' : january_offset, 
        'dst': 0, 
        'hemisphere' : jstz.HEMISPHERE_UNKNOWN
    };
};

jstz.get_january_offset = function () {
	return jstz.get_date_offset(new Date(2011, 0, 1, 0, 0, 0, 0));
};

jstz.get_june_offset = function () {
	return jstz.get_date_offset(new Date(2011, 5, 1, 0, 0, 0, 0));
};

/**
 * Uses get_timezone_info() to formulate a key to use in the olson.timezones dictionary.
 * 
 * Returns a primitive object on the format:
 * {'timezone': TimeZone, 'key' : 'the key used to find the TimeZone object'}
 * 
 * @returns Object 
 */
jstz.determine_timezone = function () {
	var timezone_key_info, hemisphere_suffix, tz_key;
	timezone_key_info = jstz.get_timezone_info();
	hemisphere_suffix = '';
		
	if (timezone_key_info.hemisphere === jstz.HEMISPHERE_SOUTH) {
		hemisphere_suffix = ',s';
	}
	
	tz_key = timezone_key_info.utc_offset + ',' + timezone_key_info.dst + hemisphere_suffix;
	
	return {'timezone' : jstz.olson.timezones[tz_key], 'key' : tz_key};
};

/**
 * The keys in this dictionary are comma separated as such:
 * 
 * First the offset compared to UTC time in minutes.
 *  
 * Then a flag which is 0 if the timezone does not take daylight savings into account and 1 if it does.
 * 
 * Thirdly an optional 's' signifies that the timezone is in the southern hemisphere, only interesting for timezones with DST.
 * 
 * The values of the dictionary are TimeZone objects.
 */
jstz.olson.timezones = {
    '-720,0'   : new jstz.TimeZone('-12:00', 'Etc/GMT+12', false),
    '-660,0'   : new jstz.TimeZone('-11:00', 'Pacific/Pago_Pago', false),
    '-600,1'   : new jstz.TimeZone('-11:00', 'America/Adak', true),
    '-660,1,s' : new jstz.TimeZone('-11:00', 'Pacific/Apia', true),
    '-600,0'   : new jstz.TimeZone('-10:00', 'Pacific/Honolulu', false),
    '-570,0'   : new jstz.TimeZone('-10:30', 'Pacific/Marquesas', false),
    '-540,0'   : new jstz.TimeZone('-09:00', 'Pacific/Gambier', false),
    '-540,1'   : new jstz.TimeZone('-09:00', 'America/Anchorage', true),
    '-480,1'   : new jstz.TimeZone('-08:00', 'America/Los_Angeles', true),
    '-480,0'   : new jstz.TimeZone('-08:00', 'Pacific/Pitcairn', false),
    '-420,0'   : new jstz.TimeZone('-07:00', 'America/Phoenix', false),
    '-420,1'   : new jstz.TimeZone('-07:00', 'America/Denver', true),
    '-360,0'   : new jstz.TimeZone('-06:00', 'America/Guatemala', false),
    '-360,1'   : new jstz.TimeZone('-06:00', 'America/Chicago', true),
    '-360,1,s' : new jstz.TimeZone('-06:00', 'Pacific/Easter', true),
    '-300,0'   : new jstz.TimeZone('-05:00', 'America/Bogota', false),
    '-300,1'   : new jstz.TimeZone('-05:00', 'America/New_York', true),
    '-270,0'   : new jstz.TimeZone('-04:30', 'America/Caracas', false),
    '-240,1'   : new jstz.TimeZone('-04:00', 'America/Halifax', true),
    '-240,0'   : new jstz.TimeZone('-04:00', 'America/Santo_Domingo', false),
    '-240,1,s' : new jstz.TimeZone('-04:00', 'America/Asuncion', true),
    '-210,1'   : new jstz.TimeZone('-03:30', 'America/St_Johns', true),
    '-180,1'   : new jstz.TimeZone('-03:00', 'America/Godthab', true),
    '-180,0'   : new jstz.TimeZone('-03:00', 'America/Argentina/Buenos_Aires', false),
    '-180,1,s' : new jstz.TimeZone('-03:00', 'America/Montevideo', true),
    '-120,0'   : new jstz.TimeZone('-02:00', 'America/Noronha', false),
    '-120,1'   : new jstz.TimeZone('-02:00', 'Etc/GMT+2', true),
    '-60,1'    : new jstz.TimeZone('-01:00', 'Atlantic/Azores', true),
    '-60,0'    : new jstz.TimeZone('-01:00', 'Atlantic/Cape_Verde', false),
    '0,0'      : new jstz.TimeZone('00:00', 'Etc/UTC', false),
    '0,1'      : new jstz.TimeZone('00:00', 'Europe/London', true),
    '60,1'     : new jstz.TimeZone('+01:00', 'Europe/Berlin', true),
    '60,0'     : new jstz.TimeZone('+01:00', 'Africa/Lagos', false),
    '60,1,s'   : new jstz.TimeZone('+01:00', 'Africa/Windhoek', true),
    '120,1'    : new jstz.TimeZone('+02:00', 'Asia/Beirut', true),
    '120,0'    : new jstz.TimeZone('+02:00', 'Africa/Johannesburg', false),
    '180,1'    : new jstz.TimeZone('+03:00', 'Europe/Moscow', true),
    '180,0'    : new jstz.TimeZone('+03:00', 'Asia/Baghdad', false),
    '210,1'    : new jstz.TimeZone('+03:30', 'Asia/Tehran', true),
    '240,0'    : new jstz.TimeZone('+04:00', 'Asia/Dubai', false),
    '240,1'    : new jstz.TimeZone('+04:00', 'Asia/Yerevan', true),
    '270,0'    : new jstz.TimeZone('+04:30', 'Asia/Kabul', false),
    '300,1'    : new jstz.TimeZone('+05:00', 'Asia/Yekaterinburg', true),
    '300,0'    : new jstz.TimeZone('+05:00', 'Asia/Karachi', false),
    '330,0'    : new jstz.TimeZone('+05:30', 'Asia/Kolkata', false),
    '345,0'    : new jstz.TimeZone('+05:45', 'Asia/Kathmandu', false),
    '360,0'    : new jstz.TimeZone('+06:00', 'Asia/Dhaka', false),
    '360,1'    : new jstz.TimeZone('+06:00', 'Asia/Omsk', true),
    '390,0'    : new jstz.TimeZone('+06:30', 'Asia/Rangoon', false),
    '420,1'    : new jstz.TimeZone('+07:00', 'Asia/Krasnoyarsk', true),
    '420,0'    : new jstz.TimeZone('+07:00', 'Asia/Jakarta', false),
    '480,0'    : new jstz.TimeZone('+08:00', 'Asia/Shanghai', false),
    '480,1'    : new jstz.TimeZone('+08:00', 'Asia/Irkutsk', true),
    '525,0'    : new jstz.TimeZone('+08:45', 'Australia/Eucla', true),
    '525,1,s'  : new jstz.TimeZone('+08:45', 'Australia/Eucla', true),
    '540,1'    : new jstz.TimeZone('+09:00', 'Asia/Yakutsk', true),
    '540,0'    : new jstz.TimeZone('+09:00', 'Asia/Tokyo', false),
    '570,0'    : new jstz.TimeZone('+09:30', 'Australia/Darwin', false),
    '570,1,s'  : new jstz.TimeZone('+09:30', 'Australia/Adelaide', true),
    '600,0'    : new jstz.TimeZone('+10:00', 'Australia/Brisbane', false),
    '600,1'    : new jstz.TimeZone('+10:00', 'Asia/Vladivostok', true),
    '600,1,s'  : new jstz.TimeZone('+10:00', 'Australia/Sydney', true),
    '630,1,s'  : new jstz.TimeZone('+10:30', 'Australia/Lord_Howe', true),
    '660,1'    : new jstz.TimeZone('+11:00', 'Asia/Kamchatka', true),
    '660,0'    : new jstz.TimeZone('+11:00', 'Pacific/Noumea', false),
    '690,0'    : new jstz.TimeZone('+11:30', 'Pacific/Norfolk', false),
    '720,1,s'  : new jstz.TimeZone('+12:00', 'Pacific/Auckland', true),
    '720,0'    : new jstz.TimeZone('+12:00', 'Pacific/Tarawa', false),
    '765,1,s'  : new jstz.TimeZone('+12:45', 'Pacific/Chatham', true),
    '780,0'    : new jstz.TimeZone('+13:00', 'Pacific/Tongatapu', false),
    '840,0'    : new jstz.TimeZone('+14:00', 'Pacific/Kiritimati', false)
};

/**
 * This object contains information on when daylight savings starts for
 * different timezones.
 * 
 * The list is short for a reason. Often we do not have to be very specific
 * to single out the correct timezone. But when we do, this list comes in
 * handy.
 * 
 * Each value is a date denoting when daylight savings starts for that timezone.
 */
jstz.olson.dst_start_dates = {
    'America/Denver' : new Date(2011, 2, 13, 3, 0, 0, 0),
    'America/Mazatlan' : new Date(2011, 3, 3, 3, 0, 0, 0),
    'America/Chicago' : new Date(2011, 2, 13, 3, 0, 0, 0),
    'America/Mexico_City' : new Date(2011, 3, 3, 3, 0, 0, 0),
    'Atlantic/Stanley' : new Date(2011, 8, 4, 7, 0, 0, 0),
    'America/Asuncion' : new Date(2011, 9, 2, 3, 0, 0, 0),
    'America/Santiago' : new Date(2011, 9, 9, 3, 0, 0, 0),
    'America/Campo_Grande' : new Date(2011, 9, 16, 5, 0, 0, 0),
    'America/Montevideo' : new Date(2011, 9, 2, 3, 0, 0, 0),
    'America/Sao_Paulo' : new Date(2011, 9, 16, 5, 0, 0, 0),
    'America/Los_Angeles' : new Date(2011, 2, 13, 8, 0, 0, 0),
    'America/Santa_Isabel' : new Date(2011, 3, 5, 8, 0, 0, 0),
    'America/Havana' : new Date(2011, 2, 13, 2, 0, 0, 0),
    'America/New_York' : new Date(2011, 2, 13, 7, 0, 0, 0),
    'Asia/Gaza' : new Date(2011, 2, 26, 23, 0, 0, 0),
    'Asia/Beirut' : new Date(2011, 2, 27, 1, 0, 0, 0),
    'Europe/Minsk' : new Date(2011, 2, 27, 3, 0, 0, 0),
    'Europe/Istanbul' : new Date(2011, 2, 27, 7, 0, 0, 0),
    'Asia/Damascus' : new Date(2011, 3, 1, 2, 0, 0, 0),
    'Asia/Jerusalem' : new Date(2011, 3, 1, 6, 0, 0, 0),
    'Africa/Cairo' : new Date(2011, 3, 29, 4, 0, 0, 0),
    'Asia/Yerevan' : new Date(2011, 2, 27, 4, 0, 0, 0),
    'Asia/Baku'    : new Date(2011, 2, 27, 8, 0, 0, 0),
    'Pacific/Auckland' : new Date(2011, 8, 26, 7, 0, 0, 0),
    'Pacific/Fiji' : new Date(2010, 11, 29, 23, 0, 0, 0),
    'America/Halifax' : new Date(2011, 2, 13, 6, 0, 0, 0),
    'America/Goose_Bay' : new Date(2011, 2, 13, 2, 1, 0, 0),
    'America/Miquelon' : new Date(2011, 2, 13, 5, 0, 0, 0),
    'America/Godthab' : new Date(2011, 2, 27, 1, 0, 0, 0)
};

/**
 * The keys in this object are timezones that we know may be ambiguous after
 * a preliminary scan through the olson_tz object.
 * 
 * The array of timezones to compare must be in the order that daylight savings
 * starts for the regions.
 */
jstz.olson.ambiguity_list = {
    'America/Denver' : ['America/Denver', 'America/Mazatlan'],
    'America/Chicago' : ['America/Chicago', 'America/Mexico_City'],
    'America/Asuncion' : ['Atlantic/Stanley', 'America/Asuncion', 'America/Santiago', 'America/Campo_Grande'],
    'America/Montevideo' : ['America/Montevideo', 'America/Sao_Paulo'],
    'Asia/Beirut' : ['Asia/Gaza', 'Asia/Beirut', 'Europe/Minsk', 'Europe/Istanbul', 'Asia/Damascus', 'Asia/Jerusalem', 'Africa/Cairo'],
    'Asia/Yerevan' : ['Asia/Yerevan', 'Asia/Baku'],
    'Pacific/Auckland' : ['Pacific/Auckland', 'Pacific/Fiji'],
    'America/Los_Angeles' : ['America/Los_Angeles', 'America/Santa_Isabel'],
    'America/New_York' : ['America/Havana', 'America/New_York'],
    'America/Halifax' : ['America/Goose_Bay', 'America/Halifax'],
    'America/Godthab' : ['America/Miquelon', 'America/Godthab']
};