_shiftHours(hours) {
      if (this._period === this.periodsArray[0]) {
        // AM
        hours = hours === 12 ? 0 : hours;
      } else if (this._period === this.periodsArray[1]) {
        // PM
        hours = hours === 12 ? hours : hours + 12;
      }
      return hours;
    }
Shifts hours value with +/- 12 depending on hour value and day period.

@param {number} hours current hours
@returns {number} shifted hours