I'm also a little concerned about corner cases near the deadline, and whether we'll meet user expectations. For instance, say I specify that the deadline is 2013-10-29. Does that mean we allow people to sign up until the end of 10/29? Is that reflected here, or are things capped at the start of 10/29? Also because we're not specifying a time for the deadline, the switchover is going to be midnight UTC, which may or may not be surprising.

I guess my inclination would be to just make `expiration_date` a `datetime` since it would remove ambiguity. But I don't know if this was already discussed with design folks.