TElPrivateKeyUsagePeriodExtension is a descendant of TElCustomExtension class.
The following paragraph is taken from RFC 2459 (Housley, et. al.), part 126.96.36.199:
«This profile recommends against the use of this extension. CAs conforming to this profile MUST NOT generate certificates with critical private key usage period extensions.
The private key usage period extension allows the certificate issuer to specify a different validity period for the private key than the certificate. This extension is intended for use with digital signature keys. This extension consists of two optional components, notBefore and notAfter. The private key associated with the certificate should not be used to sign objects before or after the times specified by the two components, respectively. CAs conforming to this profile MUST NOT generate certificates with private key usage period extensions unless at least one of the two components is present. »