As far as I can see, nothing in XEP-0237 (Roster Versioning) prevents a server from using timestamps (e.g., seconds in the Unix era) as version/sequence numbers. Would that be a popular approach? If so, we might want to add a note about it in the spec.
Peter -- Peter Saint-Andre https://stpeter.im/
smime.p7s
Description: S/MIME Cryptographic Signature
