The `listened_at` parameter for submissions is unclear. The documentation says: 

The payload should also include the listened_at element, which must be an integer representing the Unix time when the track was listened to.

But it's unclear what "when the track was listened to" means. Does it mean when the playback started? When it finished?

Last.fm specification is explicit in that it's the timestamp for when the track started playback. I think listenbrainz should follow the same, but the specification/documentation needs to be more explicit to avoid misunderstandings.