code: public DERObject toASN1Object()
      {
          ASN1EncodableVector  v = new ASN1EncodableVector();
  
          v.add(notBeforeTime);
          v.add(notAfterTime);
  
          return new DERSequence(v);
      }
docs: Produce an object suitable for an ASN1OutputStream.
  <pre>
  AttCertValidityPeriod  ::= SEQUENCE {
  notBeforeTime  GeneralizedTime,
  notAfterTime   GeneralizedTime
  }
  </pre>