code: public Date getNotBefore() {
          if (info == null)
              return null;
          try {
              Date d = (Date) info.get(CertificateValidity.NAME + DOT +
                                          CertificateValidity.NOT_BEFORE);
              return d;
          } catch (Exception e) {
              return null;
          }
      }
docs: Gets the notBefore date from the validity period of the certificate.
  
  @return the start date of the validity period.