Reference · Dates

date.valid

date.valid(any, str | null=) -> bool

What it does

Whether a value is text the date module accepts: a date "YYYY-MM-DD" that is a real day, or a date-time "YYYY-MM-DDTHH:MM:SS" with an offset (Z, +01:00). With "date" as the second argument only a date counts, with "date-time" only a date-time. Any other value, null and numbers included, gives false; nothing here is an error. Check text from a page with it before the arithmetic functions, which stop with a runtime error on anything else; or give the field the type date, which refuses bad text and date-times in a record, an annotation and a query.

Examples

print(date.valid("2026-10-27"), date.valid("2026-10-27T14:30:00Z"), date.valid("2026-02-30"))print(date.valid("2026-10-27T14:30:00Z", "date"), date.valid("2026-10-27", "date-time"))print(date.valid(42), date.valid(null), date.valid(""))

true true falsefalse falsefalse false false

Edge cases

• Only the exact form counts: "27/10/2026", "2026-1-5", a space before the text, a date-time with a space instead of T, without seconds or without an offset are all false, though date.parse reads some of them.
• The calendar is checked: 2024-02-29 is valid, 2023-02-29 is not.
• Years are 1 to 9999: "0000-12-31" is not a date.
• A fraction of a second and an offset with seconds are accepted in a date-time.
• null as the second argument is the same as leaving it out: either kind.

print(date.valid("27/10/2026"), date.valid("2026-1-5"), date.valid(" 2026-10-27"))print(date.valid("2026-10-27 14:30:00Z"), date.valid("2026-10-27T14:30Z"), date.valid("2026-10-27T14:30:00"))print(date.valid("2024-02-29"), date.valid("2023-02-29"), date.valid("0000-12-31"))print(date.valid("2026-10-27T14:30:00.123Z"), date.valid("2026-10-27", null))

false false falsefalse false falsetrue false falsetrue true

Errors

The value never fails. The second argument must be "date", "date-time" or null: anything else is a runtime error.

print(date.valid("2026-10-27", "time"))

line 1: `date.valid` takes "date" or "date-time" as argument 2, got "time"

Chaining

The value comes first in a pipeline. In a route, a check with a 400 of your own:

print(["2026-10-27", "soon", "2026-02-30"] | filter(date.valid))print("2026-10-27T14:30:00Z" | date.valid("date"))

["2026-10-27"]false

A form with date fields, and the route it posts to. A browser's type="date" sends YYYY-MM-DD, but anyone can post anything, so the route checks before counting:

route GET /leave =    <form method="post" action="/leave">        <input type="date" name="from" required/> to <input type="date" name="to" required/>        <button>Count the days</button>    </form>route POST /leave { form: { from: str, to: str } } {    if !date.valid(from, "date") || !date.valid(to, "date") { reply 400 "Give dates as YYYY-MM-DD" }    `${date.days_between(from, to)} days`}

From 2026-11-02 to 2026-11-13 the answer is 11 days; to=next week gets 400 Give dates as YYYY-MM-DD. A form without to at all does not match the pattern, and gets 400 bad request: no route pattern matches this request before the route runs.

See also

date.parse, date.of, date.parts, date.date_in, type_of

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