A format is a contract with the future
On versioning, epochs, and the obligation you take on the moment someone else writes a file you have to keep reading.
A library can be abandoned. A format cannot, not honestly — the files outlive the maintainer's interest. The moment a third party writes a file in your format you have signed something, and the only responsible question left is how long the term runs and how it ends.
We answer it with epochs. An epoch is a frozen set of semantics with a date. A reader declares which epoch it understands; a writer declares which epoch it emitted. Nothing is silently reinterpreted. Deprecation becomes a scheduled, published event rather than a surprise in someone else's build.
This is a print convention borrowed into software: the edition line. A book tells you which edition you are holding. A file should too.
[1]Epoch model adapted from the versioning discipline of long-lived archival formats.
[2]Full bibliography at the foot of the article page.