David Clinton was a small time Physics professor from Gotham City who developed the technology to travel in time as Chronos.
Perfecting his Chronosuit
David Clinton had been mocked by his time traveling theories; his peers spurned him for his claims of discovering time travel and he subsequently lost both his tenure and reputation. However, Clinton came through as he developed time traveling technology which was incorporated into a 'Chronosuit'. He used the device to collect rare relics from the past. However, he initially only took stuff that wouldn't be missed, lest he change the time-stream. After having enough from his verbally abusive wife Enid Clinton, he went several decades into the past and attempted to steal a utility belt from the Justice League's Watchtower, but he was thwarted by Green Lantern, Wonder Woman and Batman. He jumped into a time vortex, but the three League members chased him.
Trapped in the Old West
When David Clinton arrived in the Old West (circa 1879), he was jumped by men working for local crime lord Tobias Manning. After they stole his time traveling device, Manning used to steal advanced weaponry from the future and then take over the town Elkhorn. Clinton was kept behind bars for six months to teach Manning how to use the technology the crime lord couldn't understand. With the Justice League finally caught up with him and teamed up with Bat Lash, El Diablo, Jonah Hex and Ohiyesa Smith to defeat Manning and his outlaws, Clinton managed to retrieve his belt and fled in a time portal, followed by Batman, Green Lantern and Wonder Woman again.
A Dictator in his own time
David Clinton's newfound power drove him insane and became a megalomaniacal sociopath. He traveled back to his own time and asserted himself as "Lord Chronos", the undisputed master of space and time. With a new, upgraded Chronosuit, he no longer cared about time paradoxes and started collecting all sorts of relics from the past while poaching for advanced technology from the far distant future. He took over Gotham City and used the city as a big storage space to collect his memorabilia. He also made himself 'king' escorted by a very much tamed Enid now scared of Chronos's temper and mood swings, as well as put his mother-in-law in an undisclosed "thing". He also endowed a sub-group of the Jokerz with special enhancements and enlisted the clowns as "security".
Having knowledge of the future and the past, he sent his Jokerz to intercept Batman, Green Lantern and Wonder Woman when they had come after him through the time vortex. However, the Jokerz failed because the surviving JLU members (Batman II, Static and Warhawk) had been tipped off and aided the Leaguers in their escape, Chronos ruthlessly disposed of Chucko by leaving the traitor in the past, seconds before the dinosaurs were made extinct by a meteorite. Due to Chronos's tampering, the timeline started to unravel so the League and the JLU tracked him down to reprogram his belt and restore time. Chronos fled to the beginning of time to reset it and make himself a god. However, he was promptly intercepted by Green Lantern and Batman reprogrammed the time-belt. This reversed Chronos's doings, trapping him in a time loop, right before everything started: his wife undermining him.
Chronos debuted in Atom #3 (October–November, 1962), and was a creation of Gardner Fox, Gil Kane and Murphy Anderson. David Clinton was petty crook who believed his failures were a consequence of his bad timing. While serving sentence in prison, Clinton studied and trained himself to synchronize his actions with clockwork precision. When he got out of prison, he donned a flamboyant costume and dubbed himself Chronos, the Time Thief. He also developed various time related gimmicks (sundial-shaped hovering platform, razor-sharp hands, and explosive hourglasses) to meet his goals. Chronos pitted himself against the Atom (Ray Palmer), who thwarted him on every occasion. Eventually Chronos developed devices that could actually influence the local flow of time and tamper with time, such as a time machine. Chronos soon began using his time traveling abilities to become rich, but he was eventually sent to prison. He later sold his soul to Neron, who bestowed upon him the inherent ability of time traveling. Chronos ended up losing touch with timeline and dwindled into oblivion.
Whilst not considered canon to the main DCAU, a version of Chronos more similar to his mainstream comic book version was seen in the Justice League Adventures series. At first portrayed as a villain, he was soon seen in a more benevolent light as he tried to prevent an event which took the life of his brother, Robert 'Bobby' Clinton. However, he was helped by a future incarnation of himself, from the JLU/Batman Beyond era, who urged him to try and try again after continually failing to save Bobby. This began to cause significant damage to the time-stream. Eventually, he was stopped by the Flash and The Atom after they brought Bobby to his present time and he convinced David to let go of the past and move on. With that, his older incarnation was erased from history and he turned himself in. Later, whilst in prison, the League brought a young woman to visit him, who was revealed to be his future daughter from a repaired timeline named Bobbi Clinton, in honor of his deceased brother. This presumably gave him the hope to give up the title of Chronos and live a normal happy life. He might possibly be an ancestor or great-grandfather to the version of Chronos seen in the JLU/Batman Beyond era.