How far back can I backfill activities?
There is a setting when creating a challenge to turn backfilling on or off. If backfilling is turned on, then a participants previous activities can be backfilled up to 60 days. Strava activities will be automatically backfilled, up to 60 days. For manual entries, the user can change the date on the activity entry form to go back up to 60 days.
Challenge Hound restricts unlimited backfilling because it can be disruptive to other participants. For example, if you're in a six month challenge and someone joins five months in, most participants agree that the new participant should not catapult into the leaderboard. In other words, drastic changes to the leaderboard due to backfilling can be frustrating to existing participants.