An epoch is a claim
The months and the leap rules of a calendar are constrained by the sky. The year number is not. Choosing where year 1 falls is a statement about what the calendar’s community considers the beginning of the reckonable world — and it is usually a founding, a revelation, or a seizure of power.
| Calendar | Counts from | What that marks |
|---|---|---|
| Gregorian / Julian | AD 1 | The traditional birth of Christ |
| Islamic (Hijri) | 622 CE | The Hijra — the migration to Medina |
| Hebrew | 3761 BCE | A calculated date of creation |
| Korean (Dangi) | 2333 BCE | Dangun’s founding of Gojoseon |
| Buddhist | 543 BCE | The passing of the Buddha |
| Minguo (ROC) | 1912 CE | The founding of the Republic |
| Unix time | 1 Jan 1970 | A convenient round number |
| Holocene | 10,000 BCE | The start of human civilization, proposed |
Restarting the count is a political act
Because the epoch encodes a claim, changing it is a way of asserting a new order. The French Republican calendar restarted at the abolition of the monarchy, making Year I an argument rather than a date. Japan restarts its era count with each emperor, so the year number carries the reign — the current era is Reiwa.
The reverse also happens: a calendar can outlive the claim entirely. Almost nobody using the Gregorian year 2026 intends a statement about the birth of Christ, which is why the neutral abbreviations CE and BCE now sit over the same count.
The epochs that were chosen for convenience
Technical timekeeping has its own epochs, and they are refreshingly free of ideology.
- Julian Day Number counts from 1 January 4713 BCE — chosen because several astronomical cycles coincide there, giving astronomers a single unbroken day count with no negative numbers for any recorded observation.
- Unix time counts seconds from 1 January 1970, picked because it was recent, round, and slightly before the systems that needed it.
- Lunation Number counts new moons from January 1923, an arbitrary starting lunation that simply had to be somewhere.