(The prompt: The fourth age of Equestria.)
"Your most faithful student, Princess Twilight Sparkle. CE 1013."
Princess Twilight Sparkle dated her last report confidently, and set the parchment aside for sending in the morning, when her assistant woke. But then a moment passed, and she pulled the scroll open again. There, just at the end. "CE 1013."
That shouldn't bother her, it was the correct date. But Celestian Era was... well. Things had been changing, recently. Was it still appropriate to call this the Celestian Era?
What defined a new age of the world? A small smile crept across her lips as she set the scroll down and walked over to the bed, flopping exhaustedly down on top of the covers. This was exactly the sort of thing she was good with, rote recital of facts and then cautious extrapolation from that knowledge.
The first records of pony history were from the pre-Equestrian era. This time was now commonly referred to as 'the Classical era' and other monikers further back. It was a time of myths and adventure, with such powerful names as Commander Hurricane, King Platinum the Three-Hooved, and of course Star Swirl the Bearded. To hear the scribes from the time tell it, every day was a constant struggle for survival and dominance of the land that the ponies then occupied. Of course this all lead up to the events rather inaccurately described in the modern Heart's Warming Eve pageants.
It would be easy to say that's what ended the age. The founding of Equestria, planting a flag and the tribal leaders all speaking that they were at peace. Princess Twilight was quite sure that most schoolfillies would agree with that. But most schoolfillies didn't study period texts. Twilight knew that it wasn't the sort of storybook night of singing that instantly stopped all tensions between tribes. It was decades of arguments and pseudo-kingdom boundaries even inside Equestria. A particularly inquisitive pony in the royal keep of Canterlot Castle could easily see all the crenelations and sturdy stone walls that were built more for repelling an invasion than for inspiration and beauty.
So, at what point did the ponies say they were no longer in this time period?
Of course with the perspective of foreknowledge, Twilight could point to the exact year, Year One of the Equestrian Era was reckoned from the time they were said to have planted the flag. It was very likely that the reckoning was at least a few years off, but it hardly mattered now.
The second age of Equestria had started then. For a time everything was comparatively peaceful; the Heart's Warming pageants were correct about that at least. It wasn't until... Twilight scrunched up her nose, trying to remember. EE 278? That was the time that the first reports of strange magic began to appear. Hindsight made it clear that it was Discord, of course, but at the time the ponies didn't know what to make of it. Towns would disappear, and then reappear upside down. Large populations of lions and eagles would disappear, and rumors of a new combined race began flowing from the far east. The dragon migration split, for the first and only time in recorded history, and lost and confused dragons began attacking ponies just for food and sport.
There weren't many records for a while, but according to Princess Celestia, it was something like EE 350 when she and Princess Luna managed to use the Elements of Harmony for the first time, and seal Discord away. There, that was a major event. Even the ponies alive back then must have known it was a new era. Princess Celestia and Luna both took the reins of Equestria, and began rebuilding it. Luna's rebellion came a decade or two later. But even so, the Celestian Era didn't start for another fifty years.
CE 1013. Thirteen years now, after Princess Twilight had found the Elements of Harmony, and defeated Nightmare Moon on her return. It hadn't been nearly the same scale of event as Discord's first defeat, but... it could have been. If she had failed, the current era of Equestrian history would be one of eternal night. Should the Celestian Era have ended there? Would future historians mark CE 1000 as the last year of that and then... what? The Age of Harmony?
Or was that too small? Maybe it would start when the Crystal Empire returned. That was certainly a big event. Though it was not, strictly speaking, an Equestrian event. The Crystal Empire was closely allied and of course lead by family of Celestia and Luna, but it was not in their direct command.
Would it start with her coronation? Twilight couldn't help but blush slightly at the idea of being that central to history. Certainly, her coronation had been important. Not that many princesses existed, so a new one being crowned was always a huge incident. But Cadance didn't start a new age from her coronation. Also, though Twilight had created new magic, and several more spells since, Star Swirl the Bearded didn't start a new era with his research.
But so much had happened, since. The Everfree Forest had been cleaned from the world. True diplomatic relations now existed with the dragons, the Equestrian borders had grown by nearly a third in the past ten years, and of course the incident with Saddle Arabia... Twilight shook her head. She was getting off topic.
How did ponies know when something new started?
Twilight's horn glowed softly in the darkened room, as she pulled the letter over to her. She unfurled the scroll, and used the nimbus of her magic to see. The quill moved quickly, before she had time to overthink it.
CE 1013 now had a line through it, and AoH 1 was written next to it.
Twilight stared at the ink on the page. Right there, at the bottom, she had penned a new age. Was that all it took? Was it that simple?
A thousand years from now, would some scholar be laying in her bed at night, and wonder who first decided it was the Age of Harmony? Would she wonder why it was strangely ten or thirteen years offset from one of the major deciding events? Twilight smiled a little, and wrote a short post-script detailing her reckoning of the changing of the eras.
Then Twilight set down her quill, and Equestria entered its fourth age.