C# 是否向PKCS7签名CMS添加签名时间？

我正在尝试将“签名时间”属性添加到使用SignedCMS签名的文件中

private byte[] signFile(byte[] fileContent, X509Certificate2 verificationCert)
{
   ContentInfo contentInfo = new ContentInfo(fileContent);

   SignedCms signedCMS = new SignedCms(contentInfo);

   CmsSigner cmsSigner = new CmsSigner(SubjectIdentifierType.IssuerAndSerialNumber, verificationCert);

   Oid signedDate = new Oid("1.2.840.113549.1.9.5"); //oid for PKCS #9 signing time 

   signedDate.Value = DateTime.Now.ToString();

   CryptographicAttributeObject cryptoAtty = new CryptographicAttributeObject(signedDate);

   cmsSigner.SignedAttributes.Add(cryptoAtty);

   signedCMS.ComputeSignature(cmsSigner, false);

   byte[] encoded = signedCMS.Encode();

   return encoded;
}

编码时引发错误：

CryptographicException: The object identifier is poorly formatted. 

有没有关于如何正确增加签名时间的想法？我想我可能必须将签名时间转换为ASN.1编码的对象，并将其添加到

cryptoAtty

的值中。如何将日期/时间转换为ASN.1编码的对象

那很容易

cmsSigner.SignedAttributes.Add(new Pkcs9SigningTime());