All CityTins are valid in the calendar year for which they are published and expire on December 31st of that year.
Not sure which tin you have? The tin cover will have the year in which the tin is valid. The sticker on the bottom of the tin will list the expiration date, as well, including any fine print. All expiration info and fine print are also listed on all of the redeemable coasters in the tin. You've covered. (Odd as this may sound, we are unable to exchange expired tins for current editions; even if you lost your tin, even if you misplaced your tin, even if a family member gave you an expired tin. True story).