_now(dateString) {
    if (dateString) {
      return new Date(dateString);
    }
    if (!this._nowDate) {
      this._nowDate = new Date();
    }
    return this._nowDate;
  }
If dateString is provided,
return the Date it represents.
If not, will return the date when it was first called.
This is useful for us, as we want all import operations to
have the exact same date for easier later retrieval.

@param {String} dateString a properly formatted Date