I think I agree with Sven here, but for different reasons.

If the oldest possible date is twelve days ago, then okay, sure, maybe I don't care if I've heard a song in the last twelve days or not.

But here's the real problem in my opinion: At any given time, MOST of my collection will still be equally randomized when I'm using the time-based randomizer. In other words, the feature doesn't really work as intended if it's only got a 12-day memory.