"Crosscert is expired" warnings: RSA->Ed25519 identity crosscertifice apparently made in 1970?

On #22460 (moved), we found one mysterious case of link handshake failure: An RSA->Ed25519 identity cross-certificate expiring at Dec 30, 1979. This could likely be caused by passing "0" as the now value in load_ed_keys() : see comment:19:ticket:22460 .

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