Some nice methods to add on to the js date interface

js-dateMethods.js

Date.prototype.stdTimeOffset = function() {
  var jan = new Date(this.getFullYear(), 0, 1) // july and jan are currently in/out of DST in every timezone.
  var jul = new Date(this.getFullYear(), 6, 1) 
  return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset())
}

Date.prototype.isDST = function() {
  return this.getTimezoneOffset() < this.stdTimeOffset()
}

Date.prototype.getTimezone = function() {
  var str = this.toString()
  var tz = str.match(/\([A-Z]+\)/g)[0].replace(/\(|\)/g, '') // look for the timezone name in the datestring
  var tzo = this.getTimezoneOffset()  // minutes for some reason
  var neg = str.match(/-\d{4}/g) ? -1 : 1
  return { timezone: tz, offset: neg * (tzo * 60000) }
}

Date.prototype.setTimezone = function(tz) {
  var ct = this.timezoneOffsets[tz]
  if (this.currentOffset !== ct) this.setTime(+this + ct)
  this.currentOffset = ct
  return this
}

Date.prototype.timezoneOffsets = {
  "Africa/Algiers"                  : 3600000,
  "Africa/Cairo"                    : 7200000,
  "Africa/Casablanca"               : 0,
  "Africa/Harare"                   : 7200000,
  "Africa/Johannesburg"             : 7200000,
  "Africa/Monrovia"                 : 0,
  "Africa/Nairobi"                  : 10800000,
  "America/Argentina/Buenos_Aires"  : 10800000,
  "America/Bogota"                  : -18000000,
  "America/Caracas"                 : -16200000,
  "America/Chicago"                 : -21600000,
  "America/Chihuahua"               : -25200000,
  "America/Denver"                  : -25200000,
  "America/Godthab"                 : 10800000,
  "America/Guatemala"               : -21600000,
  "America/Guyana"                  : -14400000,
  "America/Halifax"                 : -14400000,
  "America/Indiana/Indianapolis"    : -18000000,
  "America/Juneau"                  : -32400000,
  "America/La_Paz"                  : -14400000,
  "America/Lima"                    : -18000000,
  "America/Los_Angeles"             : -28800000,
  "America/Mazatlan"                : -25200000,
  "America/Mexico_City"             : -21600000,
  "America/Monterrey"               : -21600000,
  "America/New_York"                : -18000000,
  "America/Phoenix"                 : -25200000,
  "America/Regina"                  : -21600000,
  "America/Santiago"                : -14400000,
  "America/Sao_Paulo"               : 10800000,
  "America/St_Johns"                : -12600000,
  "America/Tijuana"                 : -28800000,
  "Asia/Almaty"                     : 21600000,
  "Asia/Baghdad"                    : 10800000,
  "Asia/Baku"                       : 14400000,
  "Asia/Bangkok"                    : 25200000,
  "Asia/Chongqing"                  : 28800000,
  "Asia/Colombo"                    : 19800000,
  "Asia/Dhaka"                      : 21600000,
  "Asia/Hong_Kong"                  : 28800000,
  "Asia/Irkutsk"                    : 32400000,
  "Asia/Jakarta"                    : 25200000,
  "Asia/Jerusalem"                  : 7200000,
  "Asia/Kabul"                      : 16200000,
  "Asia/Kamchatka"                  : 43200000,
  "Asia/Karachi"                    : 18000000,
  "Asia/Kathmandu"                  : 20700000,
  "Asia/Kolkata"                    : 19800000,
  "Asia/Krasnoyarsk"                : 28800000,
  "Asia/Kuala_Lumpur"               : 28800000,
  "Asia/Kuwait"                     : 10800000,
  "Asia/Magadan"                    : 43200000,
  "Asia/Muscat"                     : 14400000,
  "Asia/Novosibirsk"                : 25200000,
  "Asia/Rangoon"                    : 23400000,
  "Asia/Riyadh"                     : 10800000,
  "Asia/Seoul"                      : 32400000,
  "Asia/Shanghai"                   : 28800000,
  "Asia/Singapore"                  : 28800000,
  "Asia/Taipei"                     : 28800000,
  "Asia/Tashkent"                   : 18000000,
  "Asia/Tbilisi"                    : 14400000,
  "Asia/Tehran"                     : 12600000,
  "Asia/Tokyo"                      : 32400000,
  "Asia/Ulaanbaatar"                : 28800000,
  "Asia/Urumqi"                     : 28800000,
  "Asia/Vladivostok"                : 39600000,
  "Asia/Yakutsk"                    : 36000000,
  "Asia/Yekaterinburg"              : 21600000,
  "Asia/Yerevan"                    : 14400000,
  "Atlantic/Azores"                 : -3600000,
  "Atlantic/Cape_Verde"             : -3600000,
  "Atlantic/South_Georgia"          : -7200000,
  "Australia/Adelaide"              : 34200000,
  "Australia/Brisbane"              : 36000000,
  "Australia/Darwin"                : 34200000,
  "Australia/Hobart"                : 36000000,
  "Australia/Melbourne"             : 36000000,
  "Australia/Perth"                 : 28800000,
  "Australia/Sydney"                : 36000000,
  "Europe/Amsterdam"                : 3600000,
  "Europe/Athens"                   : 7200000,
  "Europe/Belgrade"                 : 3600000,
  "Europe/Berlin"                   : 3600000,
  "Europe/Bratislava"               : 3600000,
  "Europe/Brussels"                 : 3600000,
  "Europe/Bucharest"                : 7200000,
  "Europe/Budapest"                 : 3600000,
  "Europe/Copenhagen"               : 3600000,
  "Europe/Dublin"                   : 0,
  "Europe/Helsinki"                 : 7200000,
  "Europe/Istanbul"                 : 7200000,
  "Europe/Kiev"                     : 7200000,
  "Europe/Lisbon"                   : 0,
  "Europe/Ljubljana"                : 3600000,
  "Europe/London"                   : 0,
  "Europe/Madrid"                   : 3600000,
  "Europe/Minsk"                    : 10800000,
  "Europe/Moscow"                   : 14400000,
  "Europe/Paris"                    : 3600000,
  "Europe/Prague"                   : 3600000,
  "Europe/Riga"                     : 7200000,
  "Europe/Rome"                     : 3600000,
  "Europe/Sarajevo"                 : 3600000,
  "Europe/Skopje"                   : 3600000,
  "Europe/Sofia"                    : 7200000,
  "Europe/Stockholm"                : 3600000,
  "Europe/Tallinn"                  : 7200000,
  "Europe/Vienna"                   : 3600000,
  "Europe/Vilnius"                  : 7200000,
  "Europe/Warsaw"                   : 3600000,
  "Europe/Zagreb"                   : 3600000,
  "Pacific/Apia"                    : 46800000,
  "Pacific/Auckland"                : 43200000,
  "Pacific/Fakaofo"                 : 46800000,
  "Pacific/Fiji"                    : 43200000,
  "Pacific/Guadalcanal"             : 39600000,
  "Pacific/Guam"                    : 36000000,
  "Pacific/Honolulu"                : -36000000,
  "Pacific/Majuro"                  : 43200000,
  "Pacific/Midway"                  : -39600000,
  "Pacific/Noumea"                  : 39600000,
  "Pacific/Pago_Pago"               : -39600000,
  "Pacific/Port_Moresby"            : 36000000,
  "Pacific/Tongatapu"               : 46800000
}