Timezone / Daylight Saving Time aware JavaScript class

DateTime.js

/**
 *   var d = new DateTime(2017, 3, 21, 19, 0, 0, 'JST');
 *   d.setTimezone('PST');
 *   console.log(d.format());
 */
var DateTime = (function () {

  // Constructor
  var DateTime = function(year, month, date, hour, min, sec, tz) {
    month = month - 1;
    if (!tz) {
      tz = 'Z';
    }

    var input = Date.UTC(year, month, date, hour, min, sec);
    var dtsOffset = 0;
    if (this.timezones[tz][1]) {
      dtsOffset = (this.dtsOffset[this.timezones[tz][1]])(new Date(input));
    }

    var d = new Date();
    this.localOffset = d.getTimezoneOffset() * -60000; // msec

    this.setTimezone(tz);
    this.utcTime = input - this.zoneOffset - dtsOffset * 3600000;
    this.utcDate = new Date(this.utcTime);
  }

  var p = DateTime.prototype;

  p.timezones = {
    PST: [-8, 'US'],
    CST: [-6, 'US'],
    Z:   [0,  null],
    JST: [9,  null]
  };

  p.dtsOffset = {
    US: function(localtime) {
      var y = localtime.getFullYear();
      
      // the second Sunday in March
      var marchFirstDay = new Date(y, 2, 1, 0, 0, 0).getDay(); // 0: Sun, 6: Sat
      var startDate;
      if (marchFirstDay == 0) {
        startDate = 8;
      } else {
        startDate = 15 - marchFirstDay;
      }
      startDate = new Date(y, 2, startDate, 2, 0, 0);

      var novFirstDay = new Date(y, 10, 1, 0, 0, 0).getDay();
      var endDate;
      if (novFirstDay == 0) {
        endDate = 1;
      } else {
        endDate = 8 - novFirstDay;
      }
      endDate = new Date(y, 10, endDate, 3, 0, 0);

      var t = localtime.getTime();
      if (startDate.getTime() <= t && t < endDate.getTime()) {
        return 1;
      } else {
        return 0;
      }
    }
  }

  p.setTimezone = function(tz) {
    this.timezone = tz;
    this.zoneOffset = this.timezones[tz][0] * 3600000;
    this.dtsType = this.timezones[tz][1];
  };

  p.shiftedDate = function() {
    var shiftedTime = this.utcTime - this.localOffset + this.zoneOffset;
    var shifted = new Date(shiftedTime);
    if (!this.dtsType) {
      return shifted;
    }
    var dtsOffset = this.dtsOffset[this.dtsType](shifted);
    shiftedTime = shiftedTime + dtsOffset * 3600000;
    return new Date(shiftedTime);
  }

  p.format = function() {
    return this.dateToString(this.shiftedDate());
  }

  p.dateToString = function(d) {
    var pad = function(n) { return n < 10 ? '0' + n : n };
    return d.getFullYear()
      + '-' + pad(d.getMonth() + 1)
      + '-' + pad(d.getDate())
      + ' ' + pad(d.getHours())
      + ':' + pad(d.getMinutes());
      + ':' + pad(d.getSeconds());
  }

  return DateTime;
})();