@Edent as others mentioned, this is a Gitlab specific "feature". Classic SSH keys don't have any date information. There's a thing called "SSH Certificates" that use X509 certs to if you want that pain. (Useful in other ways though.)
re: Same keys - its fine, IMO. Better to have different keys per computer. Also better to have different keys per security domain. (Personal servers vs corporate servers vs external companies.) But 'better' is relative and marginal.