Scallions (Green Onions) (cool-season allium) is classed 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
4–2 weeks before the median last spring freeze
Colorado State University Extension's CMG GardenNotes #720 groups onions with the Hardy Vegetables that may go in "As early as 2-4 weeks before the date of the average last spring frost," and the same publication's Table 1 Cool Season Vegetable Planting Guide lists "Onions, green" among those cool-season crops, so the 2-to-4-week pre-frost span is used directly; no other extension source located gave a later-starting green-onion spring rule, so nothing needed reconciling.
Fall planting
8–4 weeks before the median first fall freeze
Purdue Extension HO-186-W Table 2 lists green onion among cool-season plants for late-summer or fall planting to be planted "at least 4-8 weeks before first fall frost," which encodes as a window running from eight weeks down to four weeks ahead of the first fall freeze; it was the only extension source found anchoring a green-onion fall sowing to frost, so it stands unreconciled.
Variety assumption: Assumes non-bulbing bunching types or true onions pulled young; Colorado State's Table 1 gives 60 typical days to harvest for "Onions, green" versus 110 for dry onions, and this entry encodes only the green-onion crop.