Guardians of Time in D&D Lore?

Yeah, the god Cyndor of Greyhawk leads an order of time guardians.

There are also Time Dragons in one of the late Dragon Magazine articles (I'm thinking 357-- it's listed in one of my monster index topics, I think the Inner Plane one)

Generally, the true "guardians of time" are gods with Time in their portfolio, such as Irunean in Violetspace (Violet Dawn campaign setting), Cyndor of Greyspace, Chronus the Protogenos, Chronos the Titan (though he probably doesn't have much control over it anymore), etc.
In Violet Dawn, Irunean is also the god of space (dimensions), so he also controls planar travel to and from Violetspace (not surprisingly he is by far the most powerful of Temulea's children, at least within the Violet Dawn setting. The four Elemental Lords of Violetspace, assuming they are merely Avadnian equivalents of the Torilian/Planescape Elemental Lords *which is much easier to do than trying to fit an additional elemental god on each elemental plane*, would obviously be far more powerful deities overall, since Irunean is probably just an Intermediate power, but their power over Avadnu is not all that great, especially since it has been sealed off.)
The powers of time and magic generally work to severely limit Chronomancy, both divine and arcane, not only on the planes but also on the Prime. Of course, on the Prime, the deities of time and magic can only control the use of Chronomancy within their own crystal sphere(s).

Unlike Chronomancy, there are probably deities which can control or influence other crystal spheres outside their own-- with the aid of the native powers. At least, that's how I have it in my campaigns-- Ptah and Celestian being among the few who can do this (in fact, the way I incorporated Violet Dawn into mainstream D&D, Ptah and Celestian aided Irunean in creating the Demiplane of Doors and sealing Violetspace off from the rest of the Multiverse-- their motivations of course being different-- Irunean wanted to save Violetspace from The Void, while Celestian and Ptah wanted to quarantine everything The Void touched from the rest of the Multiverse. The Demiplane of Doors was a concession with Irunean and the other Avadnian powers, allowing a tiny amount of divine energy to flow back into Violetspace, and allowing a tiny amount of worship energy to trickle back into the Outer Planes in order to sustain the Avadnian pantheon.)

A chronotyryn appeared in the Downer comics in Paizo-era Dungeon Magazine. Described as a "time lord from the far future," it plays a role in a complex game between planar high-ups, with the prize being the heart of a newborn god.

One minor observation is that chronotyryns are supposed to be from Acheron, and they seem to continue the "birds with too many legs" theme that the achererai started. I thought that maybe the mysterious, long-extinct hassitor, with their many-legged undead fortresses, might have had a similar appearance.