Zinnia (flower) is classed tender in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Transplant outdoors
from the median last spring freeze to 2 weeks after
NC Cooperative Extension (Pender County) names zinnias among the large-seeded flowers sown indoors to transplant outdoors after the threat of frost, giving a 14-day set-out window opening at the last spring freeze; UW-Madison also mentions setting young transplants out, but attaches no frost anchor to that clause, so it does not move the window.
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
NC State's Extension Gardener Plant Toolbox says to sow zinnia seed directly in the ground after the last frost date and UW-Madison Extension independently places direct sowing after the danger of frost has passed in spring, so the two agree on the anchor and the window opens at the median last spring freeze and runs the standard 14-day span.
No start seeds indoors window: No indoor_start window is encoded: the UMN zinnia page used in the earlier draft is not cited because it never uses the word frost and fixes its outdoor date by soil temperature (70 degrees F), and UW-Madison's companion clause about starting seed indoors 4 to 6 weeks earlier is measured from the direct-sow date rather than given as its own frost-anchored range, so no unambiguous offset from the median last freeze follows.
Variety assumption: UW-Madison additionally conditions outdoor seeding on the soil warming and gives a 65 to 70 degrees F germination temperature, so this frost-anchored window can open before the soil is actually warm enough in a cool spring.
"Seed can be direct sown in the garden or landscape after the danger of frost has passed in spring, and it can also be started indoors 4 to 6 weeks earlier."
"Large seeded flowers and vegetables are the easiest to grow and include cucumbers, zucchini, squash, melons, cosmos, marigolds, nasturtium, zinnias, and sunflowers. To get a head start on the season, seeds of these varieties can be sown indoors in mid-March to transplant outdoors after the threat of frost."