koyomi

.cal — dates: closing and payment days, business days, legal periods.

koyomi writes a date as a contract or a law says it: thirty days after the invoice, moved to the next business day. koyomi check computes it on every day of the declared range and checks each claim on every one of them; only a file that passes compiles, into TypeScript, Python, Go, Rust and SQL, and into a Wasm module that any host calls with one JSON object.

koyomi's documents are in the repository: README, the reference, the generated code, the codes of its diagnostics.

Comparing two versions

koyomi diff compares the answers of two versions: of a calendar on every day both know, of a dates file on every input both take. Here an office's calendar closes on its founding day, renames a bank holiday, knows 2027 too and takes an offset:

$ cd crates/koyomi/tests/diff
$ koyomi diff office.cal office_v2.cal
koyomi diff: calendar office v1 → v2
  old: office.cal
  new: office_v2.cal

compared: 2026-01-01..2026-12-31 (365 days, the days both versions know)

open or closed changed (1 interval, 1 day):
  2026-06-10: open → closed (Founding day)

closed in both, for other reasons (1 interval, 1 day):
  2026-05-04: Early May bank holiday → May Day

the days known: 2026-01-01..2026-12-31 → 2026-01-01..2027-12-31
offset: none → +01:00

every other day of 2026-01-01..2026-12-31 gets the same answer.

Each side is a path, a path at a git revision (calendars/office.cal@origin/main, the file as it is on that branch), a file of a dependency at the version the lock pins (@calendars/office.cal), or a file of another version of that dependency in the store (@calendars@1.0.0/office.cal). The exit code is 0 when nothing differs, 1 when something does, and 2 when the two cannot be compared: a calendar beside a dates file, or a side that does not pass koyomi check. --format markdown writes the same in Markdown, and --format json gives every interval.