Summer squash & zucchini (warm-season vine crop) is classed very 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
Start seeds indoors
2–1 weeks before the median last spring freeze
Sow in pots 3–4 weeks before the outdoor window opens (UNH 3–4 weeks; Missouri G6570 3–5 weeks).
Transplant outdoors
2–4 weeks after the median last spring freeze
UNH: set out 2 weeks after last frost; Purdue very-tender class agrees.
Direct-sow outdoors
2–4 weeks after the median last spring freeze
Purdue: very tender — at least two weeks after the average last frost. Penn State earliest sowing May 15 (+14 days) agrees.
Seed germination soil temperature: optimum 95°F (optimum range 70–95°F); minimum 60°F, maximum 100°F. Purdue HO-186-W Table 3 (Purdue lists a single 'Squash' row.). Cold soil below the minimum rots seed — a reason windows and soil readings both matter.
Variety assumption: Assumes an average zucchini-type variety (about 50–60 days from seed; Penn State lists 60).
Hardiness classes: hardy = plant 4-6 wk before last spring frost; semi-hardy = 2-4 wk before; tender = after average last frost; very tender = at least 2 wk after. Table 2: fall planting at least 4-8 wk before first fall frost.
Earliest/latest sowing dates for a May 1 last-frost / Oct 1 first-frost baseline; offsets derived by date difference from May 1. Also days-to-maturity figures.