区分已过期的PGP子项与永不过期的PGP子项 - Java bouncycastle

I am using bouncycastle.openpgp library to get the validity dates of a PGP key. A PGP key can be assumed to be never expiring if the key.getValidDays() == 0. How to differentiate between a once valid and now expired key vs a never expiring one? I am using bouncycastle.openpgp library to get