There needn't be a specific date on the ticket... ZED tickets are valid for a certain period (3 months I think, but check this), so you're good for travel up to the exp date.

In fact, ZED tickets are particularly great if you're not on a fixed schedule: it gives you maximum flexibility at no additional cost whatsoever. If you want to fly out, just check if there's a seat, list and go. Can't be better than this, for a gypsy, can it? ;-)