TElPGPKeyExpirationTimeSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 22.214.171.124.:
«The validity period of the key. This is the number of seconds after the key creation time that the key expires. If this is not present or has a value of zero, the key never expires. This is found only on a self-signature.»