Snapshots

A copy of the whole league — every match, every game, every score, and the rosters as they stood — is kept every 10 minutes on a match day between 11:00 and 21:00, again straight after anything changes, and whenever you press Take snapshot now. Right now: outside the window — outside it, only changes and manual snapshots are recorded, which is all there is to record. The most recent 500 are kept.

Restoring puts the matches, games, scores and bracket back exactly as they were, removes any match created since (a bracket built later, for instance), and re-rates the season from the restored scores. Rosters are recorded but never rewritten, so anyone signed up since stays. The state you are restoring away from is snapshotted first, so a restore can itself be undone.