Most authors invent a world and then tell a story set inside it. Tolkien did something rather different. He invented a world and then wrote its history - not just the broad strokes, but the granular, meticulous, almost compulsive detail of exactly when things happened.
Pick up a copy of The Lord of the Rings and turn to the back. Past the genealogies and the language appendices, you'll find Appendix B: The Tale of Years. It's a chronology of Middle Earth spanning thousands of years, broken down not just by year but, for the critical events of the War of the Ring, by individual day. The 13th of January, Third Age 3019: the Fellowship enters Moria. The 15th: Gandalf falls on the Bridge of Khazad-dûm. The 26th of February: the Fellowship breaks at Amon Hen. It reads less like a novelist's notes and more like a historical record.
This wasn't an afterthought. Tolkien spent years working and reworking the internal chronology of The Lord of the Rings to make sure everything was consistent - that a character could physically be where the story needed them to be, that the phases of the moon matched the narrative, that the distances between locations made sense given the time allowed. He built a world with the rigour of a scholar because he was one.
The calendars are perhaps the most extreme example of this. Appendix D contains a full working calendar for the Shire, complete with its own month names, leap year rules, and a detailed explanation of how it relates to the calendars of Gondor and Rivendell. The Shire calendar has twelve months of thirty days each, plus five special days outside the months - and Tolkien worked out the leap year arithmetic carefully enough that fans have since built working simulations of it. The Elvish calendar of Rivendell runs on a six-day week. Gondor's calendar tracks time from sunrise to sunrise rather than midnight to midnight. None of this appears in the main story. It's there because Tolkien felt it needed to exist.
The 25th of March is the most famous date to come out of all this work. In the Shire Reckoning, it's the day the One Ring was destroyed in the fires of Mount Doom, Sauron was defeated, and the War of the Ring ended. Tolkien chose the date deliberately - the 25th of March was the traditional start of the legal year in England before 1752, and also carries Christian significance as the date of the Annunciation. The themes of sacrifice and what Tolkien called "eucatastrophe" - the sudden, joyous turn - run through the whole legendarium, and he chose a date that carried real-world weight to mark its culmination.
The Tolkien Society has observed Tolkien Reading Day on the 25th of March every year since 2003, specifically because of that date's significance in the story. It's a neat example of how Tolkien's internal calendar work ended up having real-world consequences decades later.
This kind of detail is part of why Middle-earth works on OpenFiction in a way that most fictional universes can't quite match yet. The dates in Appendix B are precise enough to place events on a real calendar. The 15th of March, the 25th, the 22nd of September - these are all dates we can surface in the This Day in History feature because Tolkien did the work of specifying them exactly. When you browse to the 25th of March on OpenFiction, you'll find the destruction of the One Ring sitting alongside Tolkien Reading Day, both pointing back to the same source.
Most fantasy worlds have timelines. Very few have calendars detailed enough to have their own leap year rules. Tolkien's obsession with getting the dates right is exactly why, seventy years after publication, we can still mark the anniversary of events that never happened - and feel, somehow, that we should.