Swiss chard (cool-season leafy green) is classed semi hardy 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
Direct-sow outdoors
2 weeks before the median last spring freeze, up to the freeze date
Purdue: semi-hardy — 2–4 weeks before the last frost; Penn State's earliest sowing Apr 17 (−14 days) is the later start and is used.
Seed germination soil temperature: optimum 85°F (optimum range 50–85°F); minimum 40°F, maximum 95°F. Purdue HO-186-W Table 3. Cold soil below the minimum rots seed — a reason windows and soil readings both matter.
Variety assumption: First cutting about 50–60 days from seed (Penn State lists 55); tolerates summer heat better than spinach.
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.
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.