For an Empire in the midst of Time itself, a founding date would be incoherent. The Empire of Lun both will and has always existed and is, or could be, ever-present in any time at any place from the beginning of history up until its end and possibly even after (or before) that.
If we look instead for a nexus or central point, a vector of collection which forms the core of the experience of the Empire and give it its name, then we should look for the ancient, (or possibly future-distant) river of Lun and the Golden barges thereof.
A long, long time ago, (or to-be), the river Lun was (or will be) home to a still-older race, for our purposes; Turtle-Men. (They wouldn’t use that word, they would consider us Turtle-Apes, possibly, if they considered us at all)
The Turtle-Man Philosophers of Lun had a great deal to consider and a great deal of time in which to consider it.
Philosophers before they were magicians (they would say they fundamentally still are philosophers, who have picked up a few ancillary skills) they learnt thought before language, magic before technology, calculus before arithmetic, basic wave-functions before the names of colours and regret before violence
They picked up violence eventually, they had to, they were turtle people and pretty much anything smart and ruthless kept trying to turn them over on their backs
The Empire of Lun began, essentially as an exercise in self-defence. Turtles don't know how to posture and, because they look like a bunch of extremely-slow non-violent philosophers who are sitting on a prime piece of regularly-flooding arable land plus excellent communication channel and because they are delicious and because they are rumoured to be rich.
And yes, they were/are extremely, insanely rich in golden sand and river pearls, but the philosophers of Lun care little for such trinkets.
And no you can't have any.
Because you asked for it. When you learn not to care for wealth at all then you will be wise enough to possess it.
Like us, the Philosophers of Lun.
Non-violence is all very well but it’s not nice to be turned over a cooked in your own bones by some animal that won't live even thirty years and is only intelligent through some kind of insane fluke of genetics anyway.
So the Philosophers of Lun learnt civilisation largely backwards, starting with vast universal principles and from that trying to work out how, for instance, a sword works, or how to build a wall, or what 'explosion' means in real terms of maybe taking someone's head off before they turn you over on your back.
it is (or will be, or was,) unpleasant for the Philosophers off Lun because they are/were extremely well aware that the entire universe is doomed to a slow death of entropy and every time you, for instance, cave in someone’s head with a golden sceptre topped with a gigantic pearl wrapped in silvered iron barbs, or blow up their capital ship and sink it into the archipelago of Lun (which is made up almost entirely of sunken navies of people who wished to invade the Philosophers of Lun), either way, you are increasing entropy and hastening the death of the universe.
Not by much, just a little bit, but it still counts.
And magic is very handy with that because you don't even really need to get up to do what you need to do, and it's not as if magic is hard, it's just philosophy but.. less, worse... applied philosophy.
(But magic is entropic, and therefore to be regretted).
And slowly (very slowly) but surely, the Philosophers of Lun got better and better and better at murdering the shit out of people who intended to murder the shit out of them
How did the Philosophers defend their river?
With turtle ships, big ones, ships big enough to carry cities.
And also with giant turtles.
(No-one is sure if the gigantic island-turtles of Lun are just a different form of the philosophers of Lun, maybe they just get that way when they are very old.
Maybe it’s a gender thing.
No-one really know what gender the Philosophers of Lun are, they say its irrelevant anyway).
If you can move your fortresses then every war is a defensive war, and the defensive is always the strongest form of war (They were, or will be, really excellent siege engineers.)
And the barges and the cities of Lun were/are encrusted with gold.
Partly the natural riches of the river, because the Philosophers rarely spent anything and lived a long, long time, partly because when someone comes to rob your house and you kill them, they tend to leave their stuff behind, and sometimes they robbed another house before yours.
So, like their fortified barges and like the cities islands on the shells of the gigantic turtles, the Philosophers themselves become more and more encrusted with gold, with pearls, with gems of fire and dangling platinum bells and mosaicked shards of lapis lazuli and jade.
Not, you understand, because they were into it.
Just to illustrate exactly how little wealth meant to them. Only when you seriously, totally encrust yourself with gold and jewels to you really test your own purity and will to rise above the temptations of the world. You really have to go all out with the gems.
You will also need Crab-Men to carry out all the boring brute work that you can't do as you are too busy working out the ballistics of a gigantic brass cannon built into the shell of a gigantic glyptodont, and to row the turtle ships around and whatever.
Dangerous, radical, fast-moving Pangolins to deal with all the day-to-day things that happen at the speed of falling leaves.
Luckily the Pangolin-Men don't live long enough to pick up much really dangerous magic or science, if they need a dangerous spell you can just carve it into the back of a Crab Man and send that with them, the crab-men are loyal so they always come back and without them the spell doesn't work.
But all of this, the wealth, the knowledge, the gigantic Hermit Crabs and the beautiful river of Lun didn't make them happy. Not at-all.
Yes - you can wipe out most of species and leave a very powerful shall we say "suggestion" to the rest to leave you the fuck alone till the end of time
Yes you can grow hermit crabs to gargantuan size, build fortified siege-cities on their backs and send them creeping slowly through the core of your opponents population base vomiting death from bronze mortars
But in a million years or so that species will be extinct, and then a new one will evolve and you will have to do the whole ghastly business all over again.
And you can use magic to move planets or planes, but it seems that something is always happening somewhere and nobody wants to leave you alone to ring your platinum bells in your golden tower on your gigantic river turtle.
So the Empire of Lun found the one place that no-one would ever follow them, the one place where they could always keep watch and make sure that no-one, ever, ever could reach you without your knowledge.
They took their Empire into the dimension of Time itself, the Temporal Prime, the para-plane where the lifelines of every deciding being in a reality stretch out in near-infinite silver strands deep, deep into the future and to the end of all life, and deep into the past and to life’s origin, and past that, where the silver strands peter out and only the dim, ghostly sliver fog of un-deciding physical existence flows continually, slowing down like a widening river without life to observe it, until somewhere that, too ends.
There, amidst the epic skein of silver threads, swim river-turtles of Lun, with golden cities on their backs, and the fortified turtle-ships of Lun, driven by the oars of the labouring crab-man legions. There they shoulder the storms of time.
You don't need to exert any physical effort to move on the Temporal Prime, but you need to think you are, the thought linked to the memory of movement in the body creates real movement. The turtles think they are swimming, the Crab Men think they are rowing, so they are.
Gravity on the Temporal Prime is whatever you want it to be so the golden Empire-Fleet of Lun moves in a cloud of ships bothered only occasionally by the cylindrical castles of rotating bone, crewed by skeletons, the Lich-Craft whose immortal Captains can't be bothered with the long wait between star systems and decide to take a shortcut through time itself, maybe hop a little further up their own death-line.
The Philosophers don't like this at all, but the Space-Liches don't care about ruling Time Itself, they like the material world, they put quite a bit of effort into staying there after all.
And there the Empire rests, eternal, timeless, free from entropy at last, able to travel throughout the entire history of material things as they wish and, most importantly, TO STOP ANYONE ELSE FROM DOING THE SAME THING.
You can't have some fucking _monkey_ learning Time Magic and fucking about with their own timeline, or their friends timeline, or their planets timeline, what if they create a Time Storm, what if they paradox a planet out of existence, what if they get into Time itself?
Then its going to be the same terrible situation it was in Lun, but infinitely worse.
No-one ever records the secrets of Chronomancy, partly out of the same introverted egotism of all wizards and partly because if it gets out, an intelligent Pangolin riding a Crab-man embossed with gems and carved with spells will appear before the conception of anyone who does and castrate their parents.
And if that doesn't work, Golden Barges in the sky, and the Philosophers themselves
I ripped off the Temporal Prime from Loren Colemans AD&D ‘Chronomancer’ book for TSR, which is wonderful.