As I was typing Journey
.
It may well be time for your earliest sowings, Trunkman. But, USDA hardiness zones are for winter hardiness of plants. That is why you see the zone designation on trees and shrubs and such at the nursery. They are to answer the question: "Will this plant live through my winters?"
Vegetables sown from seed are usually not grown through the winter - altho' spinach is one that
can be. (Especially, if you go by the name of Journey and keep your spinach happy under plastic through the winter
.) Still, you aren't concerned now with the question of whether spinach plants can survive the coldest weather of mid-winter but whether spinach seeds can sprout at the soil temperatures right now or soon.
Of the garden vegetables, only lettuce germinates quicker at low temperatures than does spinach. And, lettuce and spinach seed will begin to grow before even radish and turnip seed are warm enough to sprout! Onions? Yes, they are right in there with that group.
More important by far than winter hardiness zone -- is, when is your last frost date? Spinach seed should be fine at 2 or even 3 weeks
before that date.
Steve
! 
