detect_timezone.js

// Taken from https://bitbucket.org/pellepim/jstimezonedetect/raw/default/detect_timezone.js

/*version 2012-05-10*/

/**
 * Namespace to hold all the code for timezone detection.
 */
var jstz = (function () {
  'use strict';

  var HEMISPHERE_SOUTH = 'S',
      HEMISPHERE_NORTH = 'N';

  /**
   * Gets the offset in minutes from UTC for a certain date.
   * @param {Date} date
   * @returns {Number}
   */
  function getDateOffset(date) {
    var offset = -date.getTimezoneOffset();
    return (offset !== null ? offset : 0);
  }

  function getJanuaryOffset() {
    return getDateOffset(new Date(2010, 0, 1, 0, 0, 0, 0));
  }

  function getJuneOffset() {
    return getDateOffset(new Date(2010, 5, 1, 0, 0, 0, 0));
  }

  /**
   * This function does some basic calculations to create information about
   * the user's timezone.
   *
   * Returns a key that can be used to do lookups in jstz.olson.timezones.
   *
   * @returns {String}
   */
  function lookup() {
    var januaryOffset = getJanuaryOffset(),
        juneOffset = getJuneOffset(),
        diff = (getJanuaryOffset() - getJuneOffset());

    if (diff < 0) {
      return [januaryOffset, true, HEMISPHERE_NORTH];
    } else if (diff > 0) {
      return [juneOffset, true, HEMISPHERE_SOUTH];
    }

    return [januaryOffset, false, HEMISPHERE_NORTH];
  }

  /*
   * The values in the returned list are as such:
   *
   * First the offset compared to UTC time in minutes.
   *
   * Then a flag which is FALSE if the timezone does not take daylight savings
   * into account and TRUE if it does.
   *
   * Thirdly an optional {S,N} signifies that the timezone is in the northen
   * or southers hemisphere, only interesting for timezones with DST.
   */
  return lookup;
}());