tz_in_sharepoint.js

var tzWebOffsetPromise = function (timestamp) {
  // returns a promise for the adjusted timezone for the supplied timestamp
  return $.ajax({
    url: _spPageContextInfo.siteAbsoluteUrl + "/_api/Web/RegionalSettings/TimeZone",
    type: "GET",
    headers: {
      "Accept": "application/json;odata=verbose",
    }
  }).then(function (tz) {
    var isDST = function (date) {
      // see http://stackoverflow.com/a/11888430
      var stdTimezoneOffset = function (date) {
        var jan = new Date(date.getFullYear(), 0, 1);
        var jul = new Date(date.getFullYear(), 6, 1);
        return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset());
      }
  
      return date.getTimezoneOffset() < stdTimezoneOffset(date);
    }
    
    // description is a string like:
    // "(UTC+01:00) Amsterdam, Berlin, Bern, Rome, Stockholm, Vienna"
    // and we only want the actual offset:
    var baseOffset = tz.d.Description.slice(4, 10).replace(':', '');
    var val;
  
    // check if the timestamp is in DST
    if (isDST(new Date(timestamp))) {
      // if it's currently DST, then we need to add an hour to the
      // offset. So parse it, add an hour
      //     ['+0200' => 200    ]  [DST]
      val = (parseInt(baseOffset) + 100).toString();
      // if val is less than 1000 then prepend a zero.
      // and then build the corresponding tz offset string again.
      //     [ offset sign (+/-) ]    [the value, padded to 4 digits]
      return baseOffset.slice(0, 1) + ((val < 1000 ? '0' : '') + val);
    }
    return baseOffset;
  });
}

var date = '2010-10-10T10:00:00.000';
tzWebOffsetPromise(date).then(function (offset) {
  var adjustForTz = function (val) {
    // IE and FF does not parse datetime strings with
    // tz in them, so manually adjust:
    var tz = offset;
    var tzData = {
      sign: tz.slice(0, 1) === '+' ? -1 : 1, // invert to adjust
      hours: parseInt(tz.substr(1, 2), 10),
      minutes: parseInt(tz.substr(3, 2), 10)
    };

    var dt = new Date(val + 'Z');
    dt.setUTCHours(dt.getUTCHours() + tzData.sign * tzData.hours);
    dt.setUTCMinutes(dt.getUTCMinutes() + tzData.sign * tzData.minutes);
    return dt;
  }
  
  return adjustForTz(date);
});

tzWebOffsetPromise('10-10-2010 10:00').then(function (correctedOffset) {
  console.log(new Date(timestamp + correctedOffset));
})