Parses cron expressions into recurring Tempo.Interval.t/0 values — the same first-class recurrence that Tempo.RRule.parse/2 and native ISO 8601 repeating intervals produce.

Lets Tempo consume any cron-configured schedule (Oban, Quantum, system crontab) without rewriting it in another vocabulary. Once parsed, materialise it like any other recurrence — Tempo.to_interval/2 with a :bound — or supply :from to anchor the occurrences.

Supported formats

• 5-field POSIX: "minute hour day-of-month month day-of-week".

• 6-field (seconds-first): "second minute hour day-of-month month day-of-week". This is the variant used by Quantum and other Elixir schedulers.

• 7-field (with year): "second minute hour day-of-month month day-of-week year". A single concrete year is converted into an UNTIL limit; a year list or range (2025,2027,2029) becomes a bymonthday-style byyear filter bounded one past the last listed year.

Field grammar

Each field accepts:

• * — every value in the field's range.

• N — a single integer.

• N,M,O — a list.

• N-M — an inclusive range.

• */S — every S starting from the field's minimum.

• N-M/S — every S within the range N..M.

Day-of-week accepts SUN–SAT (case-insensitive) as synonyms for 0–6. Sunday is both 0 and 7 (cron convention); internally converted to RFC 5545's 7 (Sunday last). A day-of-week step (N/S, */S) is expanded in cron numbering — Sunday = 0, the week start — then mapped to RFC, so 0/3 is Sun, Wed, Sat and */2 is Sun, Tue, Thu, Sat. A bare-day step runs to the end of the week and does not wrap (5/2 is Fri, Sun — not Tue).

Month accepts JAN–DEC (case-insensitive) as synonyms for 1–12.

Shortcut aliases

Standard cron aliases are supported:

@reboot is not supported — it is a system-startup hook, not a time expression.

Vixie-cron extensions

• L as day-of-month — last day of the month → bymonthday: [-1].

• NL as day-of-week (e.g. 5L) — last weekday-N of the month → byday: [{-1, N}].

• N#K as day-of-week (e.g. 5#2) — Kth weekday-N of the month → byday: [{K, N}].

• W as day-of-month (e.g. 15W, LW) — the nearest weekday to that day, never crossing a month boundary → bymonthday_nearest. A Saturday snaps back to Friday, a Sunday forward to Monday, and 1W on a weekend clamps forward into the same month. Only valid on a single day, never a list or range.

POSIX day-of-month OR day-of-week

When both dom and dow are restricted to plain lists (13 * 5 — "the 13th or any Friday"), POSIX cron matches the union, not the intersection. Tempo honours this via a daily cadence carrying a bymonthday_or_byday union filter, so 13 * 5 fires on every 13th and every Friday — not only Friday-the-13ths. A Quartz extension in either field (an ordinal day-of-week 5#2/5L, or a nearest-weekday 15W) opts out and keeps the AND-composing interpretation.

Not supported (AST gaps)

• @reboot — not a time expression.