# X_FLAG_EXPIRY_INVALID

## Means

a temporary flag has no usable expiry date

## Typical cause

`expiresAt` absent, or not an ISO-8601 date or zoned date-time naming a real day (`'December 1, 2026'`, `'2026-02-30'`, `'2026-12-01T00:00:00'`) — reachable from a store snapshot or plain JS; in TypeScript the union already refuses it

## Fix

set `expiresAt` to an ISO-8601 date such as `'2026-12-01'` in `defineFlag({ key: '<key>' })`

[Flags](/error-codes#flags)