There were those who mastered the science of time who had encountered the Justice teams of their parellel planets. For instance, Professor Alpheus Hyatt and Professor Malachi Zee more than once assisted the Atoms and their teammates in the League and Society. While Hyatt first became acquainted with Professor Ray Palmer while both taught at Ivy Univesity, Zee was noted for being one of the four founders of the Time Trust when he had met Al Pratt's costumed identity. And it was as Ray and Al's alter egos, the Atom of Earths One and Two, who repeatedly assisted the prolific professors while utilizing their respective time pool and time machine to navigate through the channels of the time stream.
Then there was the nefarious Doctor Phineas Quayle (aka Doctor Anomaly) and Doctor Doome who presented these heroes new quandries. Quayle had used his invention to peer into a future he found depraved, feeling that those responsible were the members of the Justice League of American. Doome simply wished to plunder his own century for the additional resources needed to perfect his time rods, only to come into conflict with Law's Legionnaires. While Anomaly retreated to the past when he was me with defeat, Doome allegedly journeyed to the far future, although no one knows for sure as his machine exploded after he last used it.