code: function fromData(aData) {
        var propsToCopy = ["weeks", "days", "hours",
                           "minutes", "seconds", "isNegative"];
        for (var key in propsToCopy) {
          /* istanbul ignore if */
          if (!propsToCopy.hasOwnProperty(key)) {
            continue;
          }
          var prop = propsToCopy[key];
          if (aData && prop in aData) {
            this[prop] = aData[prop];
          } else {
            this[prop] = 0;
          }
        }
      }
docs: Sets up the current instance using members from the passed data object.
  
  @param {Object} aData               An object with members of the duration
  @param {Number} aData.weeks         Duration in weeks
  @param {Number} aData.days          Duration in days
  @param {Number} aData.hours         Duration in hours
  @param {Number} aData.minutes       Duration in minutes
  @param {Number} aData.seconds       Duration in seconds
  @param {Boolean} aData.isNegative   If true, the duration is negative