This post actually puts to rest (and confirms) a theory that my coworker had when I thought I had a truly unlimited metrocard. Basically, I bought an unlimited

This post actually puts to rest (and confirms) a theory that my coworker had when I thought I had a truly unlimited metrocard. Basically, I bought an unlimited monthly pass from a machine. By the third day of using it, I noticed that every day I used it, the expiration date displayed at the turnstile would advance a day. Obviously, I thought I had somehow become the owner of the golden metrocard that would work forever (I was admittedly optimistic, hoping it would be true). Several days later, when I had confirmed that it wasn't a fluke and shared the story of my fortunate acquisition of the "golden metrocard" my coworker suggested that perhaps the expiration date stored on the card was null, and the default behavior of the turnstile was to always show an expiration date 30 days out. Sure enough, when thirty days passed, the card stopped working, and I apparently blew the booth attendant's mind when I asked why the card wasn't working (something about a negative balance or some such, she'd never seen such a thing before). My coworker was absolutely right, the expiration date was cached on the card (as a null or some other invalid value in this case) and it was then checked against a list of actually expired cards over the network.