As mentioned, UrW seeds do not expire.
It can also be noted that autumn planting works in UrW, and as far as I've seen it works with all plants, not only those that are typically considered suitable for that usage (such as e.g. rye). Autumn planting means that you plant your crops in the autumn while the soil is still warm enough, have the plants germinate and grow, then have the above ground parts wither and die as the cold sets in. In spring these plants then have a head start and matures faster than spring planted crops (they basically start to grow immediately, even if the soil is too cold for planting).