I'm trying to find a way to tweet a countdown timer, which would require some way to dynamically update either text or an image or animation over the course of weeks to months. But really, this is just a single instance of a more general problem: is there a way to post any sort of dynamically updating content to Twitter?
It strikes me that the answer is most likely "no" with respect to regular user tweets, since you obviously can't embed arbitrary JavaScript in a tweet, and since (IIRC) Twitter caches images you tweet, at least for long periods of time. Is this perhaps possible with Twitter Cards, backed by a custom site?