Get the Current Timezone Offset in Hours in JavaScript, adjusting for Daylight Savings Time

current_timezone.js

function currentTimezoneOffset() {
      var dstPeriod = {
        2012: [new Date(2012, 2, 11), new Date(2012, 10, 4)],
        2013: [new Date(2013, 2, 10), new Date(2013, 10, 3)],
        2014: [new Date(2014, 2, 9),  new Date(2014, 10, 2)],
        2015: [new Date(2015, 2, 8),  new Date(2015, 10, 1)],
        2016: [new Date(2016, 2, 13), new Date(2016, 10, 6)],
        2017: [new Date(2017, 2, 12), new Date(2017, 10, 5)],
        2018: [new Date(2018, 2, 11), new Date(2018, 10, 4)],
        2019: [new Date(2019, 2, 10), new Date(2019, 10, 3)],
        2020: [new Date(2020, 2, 9),  new Date(2020, 10, 2)]
      };
      var today = new Date();
      var offset = today.getTimezoneOffset();
      var year = today.getFullYear();
      var january = new Date(today.getFullYear(), 0, 1);
      var januaryOffset = january.getTimezoneOffset();
      var july = new Date(today.getFullYear(), 6, 1);
      var julyOffset = july.getTimezoneOffset();
      var dstObserved = januaryOffset == julyOffset;
      var dstDiff = Math.abs(januaryOffset - julyOffset);
      if (dstObserved) {
        var period = dstPeriod[year] || [new Date(year, 2, 11), new Date(year, 10, 4)];
        if (today >= period[0] && today < period[1]) {
          offset += dstDiff;
        }
      }
      return ((offset / 60) * -1);
}

NOTE: This should work for all U.S. timezones, not sure about other regions though.