There's no specific date as to when Demon Slayer started except it took place in the Taisho Period between 1912 and 1926.
I'm going to assume that the story of demon slayer had just begun because of what the Hand Demon saying that a new era had started and he was still trapped their so I will assume the series started in 1912 after and that the Tragedy of Tanjiro's family only happened in 1910 and that he began his 2 year training after that before the story officially started.
And the chat gave it to people at different points in their lives, most before they started to do their main Villainy in their series so they can change up their plans, here's some other info on the other worlds besides Ainz since it took place a week after he transferred to the New World.
Junko, Two Years before she enters hopes peak academy.
Medusa, a year before she decides to become the DMA School Nurse.
Dio, 10 years before Part 2 and 60 years before Part 3.
AFO, 4 months before Deku enters UA.
And Muzan 2 years prior to killing Tanjiro's family and 4-5 years before his final fight.
*****
After a week, the bare bones of the Base/ Trade Hub had been formed with several of the main buildings as well as a large marketplace for goods to be shipped to and fro for purchase and trade both legal and illegal goods when it officially opens up.
And thus the the Floating Harbors were beginning to be made as shipments of Seastone pillars and Seastone Plating to be used as the base for the Harbors to keep Sea Kings from attacking and destroying them while I had bought Seastone Paddle Ships from the shop to be used as ferries from the harbors to the Trade Hub.
But since that was going to take plenty of time, it was about I put my plans into action now rather than later.
"Everyone ready?" I looked upon the 4 women before me, Pitou, Molay, Zazan and Palm as they were the only ones, excluding Molay for obvious reasons, that didn't have an issue to use Haki and gained a rather fast proficiency with at least Armament for the three of them with only Pitou having also a great talent In Observation Haki as well.
"Yes Dio-sama." The three chimera any women responded back, even though they were my lovers there was still a hierarchy between us after all.
"I'm ready as ever, but your going their dressed like that?" Molay said as she looked over my new outfit that would fit in well with the time period of Japan, if you ignored that I was a blonde haired foreigner, that stood a massive 6 and a half feet tall, I would look like I fit perfectly in the time period for what I was wearing.
(Img)
"It fits for the land we will be heading to, besides if need be i can move rather easily in this after all." I say as I fix my collar of my yukata as it showed off my neck and part of my bare chest, it wasn't completely a traditional yukata as the lower half went down into a pair of traditional Japanese baggy pants that then tucked the opening near my ankles into my socks as I wore shoes, not sandals, that fit for the time as well.
"I think it fits you well Dio-sama." Zazan said, obviously trying to curry my favor, but I do enjoy flattery, to an extent before it gets annoying.
"Tch, brown noser." Palm says as a comeback which leads both of them death glaring each other.
"Enough, it's time we go." I say before selecting who will come along with me before pressing the button and we were whisked away in a flash of light as the world of One Piece and all other worlds excluding Demon Slayer came to a halt.
…
..
.
Meanwhile in Muzan's Stronghold, the Infinity Castle.
The Demon King of the East himself was actually studying up on foreign culture and trade as well as new inventions that are being created around the world.
But he was soon disturbed by the phone nearby him, the one that helped him know his fate and decide to change his plans entirely as what he will do in the future.
He just ignored it as usually it was just other members talking and he couldn't be bothered to look at it as he had other things that have his interest more than the chat.
It's been extremely beneficial to him, he'll it offered him a way with 100% chance for him to make him entirely immune to the sunlight and all it requires for him was to do what he always did, like a natural disaster going around and ruining other peoples lives.
But then it kept vibrating.
And vibrating.
And vibrating.
And even more vibrating.
*)()()(* *SLAM!*
"WHAT?! What the hell is it you damn machine?!" he began to shout out at it before he opened it up and looked at what was so god damn important.
And what he saw frozen him in shock, before soon after his eyes glowed red as his face was even more pale with cold sweats dripping from his brow.
"…. Good thing I knew this day would eventually come, luckily I have made plans for this…. And if all else fails I have a final option…." He says as he grabs at his chest before letting go and getting up to walk out from his personal room as the castle began to take shape letting him easily traverse through.
Soon enough he came upon his own throne room where he took his spot upon the throne as one demon who had full control of the Infinity Castle played on their Shamisen.
"All Upper and Lower Moons, return to my castle immediately." He says both out loud and telepathically to all of the Moons.
He also sent out a message to all Lower Moon candidates and any and all demons of moderate strength while the remaining demons throughout the land were given one job.
Cause as much chaos throughout the land of the rising sun to make them all fear the darkness that encroaches upon the night.
"…. All of this is for my greater good…." As he began many different plans with everything to support his own personal survival, as his life means more to them then their own, if he dies then they all shall die.
A war is coming, and he has no plans on losing it, 'I maybe a coward like that bastard says, but I'll use everything to my advantage so I can survive this crisis, even if you have beaten the sun by now I have some ways to deal with you….'
'And even if I can't kill you I'll definitely be taking as much as I can from you before I fall!'
****
"Ahh, so this is the land of the rising sun, even at night it is rather beautiful." I say as at this time of year the cherry blossoms have bloomed and under the full moon the scenery was rather beautiful.
"So where to now Dio-sama?" Pitou said as she and the rest of them looked to me as to what we do.
"Hmph, we deal with that pest first, then we can enjoy our stay in this lovely country afterwards…" I say as I look off towards the distance as I hold out a compass pointing me in the direction I want to go.
"Don't worry Muzan, soon enough your end will finally come, and it won't be by the hand of the pesky Demon Slayers, HuhuhuHahahaha!" I say before smiling viciously and point my thumb at myself.
"Because it will be me! DIO!"