The TScCRLInvalidityDateExtension class represents the invalidity date extension, that provides the date on which the private key was compromised or the certificate otherwise became invalid.
The following paragraph is taken from RFC 5280, section 5.3.2:
"The invalidity date is a non-critical CRL entry extension that provides the date on which it is known or suspected that the private key was compromised or that the certificate otherwise became invalid. This date may be earlier than the revocation date in the CRL entry, which is the date at which the CA processed the revocation. When a revocation is first posted by a CRL issuer in a CRL, the invalidity date may precede the date of issue of earlier CRLs, but the revocation date SHOULD NOT precede the date of issue of earlier CRLs. Whenever this information is available, CRL issuers are strongly encouraged to share it with CRL users."