const
    szCPGUID_PRIVATEKEY_USAGE_PERIOD_Encode ='{E36FC6F5-4880-4CB7-BA51-1FCD92CA1453}';

type   PCPCERT_PRIVATEKEY_USAGE_PERIOD= ^CPCERT_PRIVATEKEY_USAGE_PERIOD;
       CPCERT_PRIVATEKEY_USAGE_PERIOD =record
         pNotBefore : FileTime;
         pNotAfter  : FileTime;
end;
      

function ext_PRIVATEKEY_USAGE_PERIOD(rgExtension:PCERT_EXTENSION;cExtension:Cardinal):string;
var     pExtension:PCERT_EXTENSION;
            szOID: LPSTR;
      cbStructInfo, i: Cardinal;
        pvStructInfo: pvoid;

begin
szOID:=szOID_PRIVATEKEY_USAGE_PERIOD;
pExtension := CertFindExtension(szOID, cExtension,rgExtension);
szOID:=szCPGUID_PRIVATEKEY_USAGE_PERIOD_Encode;
if (Assigned(pExtension) and CryptDecodeObject(X509_ASN_ENCODING or PKCS_7_ASN_ENCODING, szOID,
      pExtension^.Value.pbData, pExtension^.Value.cbData, 0, nil, @cbStructInfo)) then
  begin
       GetMem(pvStructInfo, cbStructInfo);
try
    begin
     CryptDecodeObject(X509_ASN_ENCODING or PKCS_7_ASN_ENCODING, szOID, pExtension^.Value.pbData,
                       pExtension^.Value.cbData, 0, pvStructInfo, @cbStructInfo);
         result:=FileTimeToSTRTime(PCPCERT_PRIVATEKEY_USAGE_PERIOD(pvStructInfo^).pNotBefore)+' - '+FileTimeToSTRTime(PCPCERT_PRIVATEKEY_USAGE_PERIOD(pvStructInfo^).pNotAfter);
    end;
 finally
  FreeMem(pvStructInfo);
 end;
    end;
end;