iirc
@fiatjaf has a relay somewhere that will timestamp your events, and
@Amethyst as a client will allow you to timestamp stuff manually, and will also display if and when stuff was timestamped; but in general its just an action 'someone' has to perform, ideally quickly after the original event occured; it does not matter who does it, because ultimately relies on the bitcoin blockchain and not on the person who caused the (indirect) reference to end up in said blockchain.
For example i timestamped this note at the time:
View quoted note →
Sorry i don't have a super satisfying clear cut answer for you at this time, part of my advocation for NIP-03 is to get us there; i think many people/devs are snoozing on this issue.