# dates.add-days

This is the civil-date kernel for the whole dates family: days-from-civil and
civil-from-days, converting a calendar date to a day number counted from
1970-01-01 and back. Every other dates capability imports it rather than
re-deriving the arithmetic.

No date library is used in any of the three languages, and that is a decision,
not an omission. JavaScript's Date parses "2026-09-16" as UTC midnight but new
Date(2026, 8, 16) as local midnight, so the same calendar day can come back a
day out depending on the machine's timezone, and setMonth rolls 31 January into
3 March. Python's datetime and Rust's chrono are each correct on their own but
would mean three different libraries answering the same question, which is
exactly what the parity vectors exist to rule out.

Impossible dates are rejected rather than normalised. 2026-02-30 is an error,
not 2026-03-02: a parser that silently rolls a bad date forward turns a
data-entry mistake into a plausible wrong answer.

Supported range is 0001-01-01 to 9999-12-31, the dates a four-digit ISO year can
express. Inside that range every division in the kernel has non-negative
operands, so truncating division in Rust, floor division in Python and
Math.floor in TypeScript all agree.