In corporate actions and derivatives, the expiration date is the final day on which a time-limited entitlement can be used. For a tender or exchange offer, it is the date and time at which the offer closes and no more acceptances are taken. For subscription rights and warrants, it is the last day to subscribe or carry out a warrant exercise; unexercised instruments then lapse and are removed from accounts, normally with no value. For an options contract, it is the day the option ceases to exist.
The expiration date is set by the issuer, offeror or exchange and may be extended, in which case a new date is announced. It is the market-level cut-off: an investor's own election deadline at a custodian or broker falls earlier, to allow instructions to pass up the chain. Operations teams monitor positions approaching expiry so that valuable rights are exercised or sold instead of being lost.