The Ordo Chronus, referred to as the "Guardians of History" and possibly the Timehunters (or, thanks to their responsibilities, the "Clockblockers"), are a minor Order of the Inquisition. They specialise in studying the flow of time within the Warp, and studying and preventing time travel. This must be a difficult chore, since the Warp, by its very nature, is uncontrollable and immeasurable. It's hard to prevent time travel when the very act of entering the Warp could have the unintended effect of launching you back in time to the War in Heaven.
They are perhaps best known for vanishing en masse into the warp whilst trying to study it. Theories as to what happened to them vary: it's the deepest levels of the Warp so what did you expect besides cyclical tentacle rape by illogical time paradoxes, or that they're chilling with the lost Primarchs and the Emperor in a hidden bunker, or that they ironically ended up travelling far into the future and haven't turned up yet (or somehow erased themselves from the timeline), or that they met a foul xeno in a police box who cannot be killed so instead they went on all kinds of wacky adventures battling evil time-travelling xeno saltshakers capable of exterminating fucking everything and were so omnicidal that they would make Necrons plushies in comparison. As everyone knows, they traveled into the past on a steam-powered flying train and have not been seen since.
Since then, the only thing they've accomplished is getting themselves stuck in a small war regarding the Imperial dating system, which is partly why the year 999.M41 has lasted so long- nobody actually knows what year it is any more.
Having returned in the 42nd Millennium, they started the Chronostrife, a big argument/civil war between themselves, Roboute Guilliman, and themselves again. Guilliman realized that even the Imperial Calendar has got messed up, and the Great Rift was only making it worse because now time no longer flowed at the same rate on different planets. Guilliman started measuring time on his brand new and proprietary Vigilus Time System, plonking down new followers wherever he landed, along with Primaris Marines to helpfully educate all the plebians on how it works. Basically he set the War of Beasts as the new year 1 and lets each planet measure time themselves using Guilliman's new convoluted rules and terminology, because fuck knows how fast their years are actually progressing anymore.
The Ordo Chronos are, of course, pissed. They're too busy fighting amongst themselves to bother opposing Guilliman though, because by looking at the five main versions of the Imperial Calendar (the others somehow being heretical before they even look at Guilliman's) they've "determined" the actual date could be early 41st Millennium or even sometime in the 43rd Millennium for all anybody actually remembers anymore.