# Expiry Calculator

Industry: Core

**Key point — Lengths of time from now**

A year is not a fixed number of seconds. `31536000` is a day short across 29 February.

A cookie, a token and a signed URL each want the same length of time in a different form. Type an amount and a unit, then pick a start: a date you type or drag, or Now. The four plates show the cookie `Max-Age`, the `Expires` header, the JWT `exp` and the signed-URL time. The table sets twelve monthly starts beside the number most code hard-codes. Every date and length on the page is a library result except that hand-written number.

**Now.** [`ttlFromNow`](/gmt/reference/duration/calculate/ttlFromNow/) reads the clock once and gives the length in every form, so the four plates name one instant. Past year 9999 it returns `null` for the whole result, because an HTTP date has no four-digit form. The pure date route is the same length from a start you choose, and the same start always gives the same answer.

**Cookie Max-Age.** [`totalDuration`](/gmt/reference/duration/calculate/totalDuration/) with `relativeTo` counts a year or a month from the start it is given. From Now, [`totalUnitsFromNow`](/gmt/reference/duration/calculate/totalUnitsFromNow/) does the same for seconds or any one unit, and takes a `timeZone`. A day counted in UTC is always 86,400 seconds.

**Expires.** [`formatHttpDate`](/gmt/reference/utc/format/formatHttpDate/) writes the header, and [`formatHttpDateFromNow`](/gmt/reference/utc/format/formatHttpDateFromNow/) writes it for a length from Now.

**JWT exp.** [`addUnix`](/gmt/reference/unix/calculate/addUnix/) adds the length to a Unix epoch. [`getUnixIn`](/gmt/reference/unix/get/getUnixIn/) gives the epoch for a length from Now.

**Signed-URL time.** [`addUtc`](/gmt/reference/utc/calculate/addUtc/) adds the length to a UTC time, and [`getUtcIn`](/gmt/reference/utc/get/getUtcIn/) gives it from Now.

A past `Expires` and a `Max-Age` of zero or less delete a cookie (RFC 6265). That is why Now refuses a negative length and the pure route does not.

See [a length of time counted from now](/gmt/guides/core-date-operations/durations/#a-length-of-time-counted-from-now).

**Worth trying:**

- [One year, a year earlier](/gmt/tools/expiry-calculator/?w=expiry&wa=%7B%22preset%22%3A%22year-common%22%7D) — the same call from 2026 gives `31536000`.
- [One year from 29 February](/gmt/tools/expiry-calculator/?w=expiry&wa=%7B%22preset%22%3A%22year-from-leap-day%22%7D) — ends on 28 February 2029.
- [One month from 31 January](/gmt/tools/expiry-calculator/?w=expiry&wa=%7B%22preset%22%3A%22month-end%22%7D) — the end clamps to 28 February.
- [A 90-day signed URL](/gmt/tools/expiry-calculator/?w=expiry&wa=%7B%22preset%22%3A%22signed-url-90%22%7D) — a length in days is the same from any start.
- [A cookie that is already gone](/gmt/tools/expiry-calculator/?w=expiry&wa=%7B%22preset%22%3A%22delete-cookie%22%7D) — a negative length, and Now refuses it.