code: public void setUtc(String utc) {
          if (utc == null) {
              this.utc = null;
          } else {
              if (utc.length() != 20) {
                  throw new IllegalArgumentException("Must be of the form YYYYMMDDTHHMMSS.sssZ");
              }
          }
      }
docs: Sets the ISO-8601 UTC time of the file, which
  must be YYYYMMDDTHHMMSS.sssZ or null