Bok choy (cool-season leafy brassica) 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
Start seeds indoors
5–3 weeks before the median last spring freeze
Maryland's pac-choi-scoped instruction to "start seeds indoors and plant 3 to 5-week old transplants into garden soil" supplies the 21–35 day seed-to-transplant lead, counted back from the start of the UNH-derived transplant window (day 0) as the registry's indoor-start policy requires, giving last spring frost minus 35 to minus 21 days; UNH's generic "four to six weeks" indoor period is not crop-specific and is not used.
Transplant outdoors
from the median last spring freeze to 2 weeks after
UNH Cooperative Extension states that Asian greens "like bok choi" bolt prematurely after cool exposure "so they should be set out after last frost" — a point rule, encoded per the registry window policy as the 14 days beginning at the last spring frost (0 to +14 days).
Direct-sow outdoors
4–3 weeks before the median last spring freeze
University of Maryland Extension, on a page whose planting facts are explicitly scoped to "collards, kale, mustard, turnips, and pac choi", directs spring sowing "3 to 4 weeks prior to frost date", giving last spring frost minus 28 to minus 21 days; UNH's "set out after last frost" guidance for Asian greens governs setting out transplants and is encoded as the transplant window rather than treated as a competing sowing interval.
No fall planting window: No fall window is encoded because Maryland's fall rule is a days-to-maturity calculation (time to maturity plus a 10–14 day "short day factor", counted back from the first fall frost) that yields a different offset for every cultivar across the 40–80 day maturity range Maryland states for these greens, while Minnesota Extension's fall guidance is fixed-calendar (direct seeding in July) rather than frost-anchored.
Variety assumption: Offsets assume leaf-harvested pac choi within the 40–80 day from-seed maturity range Maryland gives for this group of greens; both UNH and Minnesota Extension warn that this crop bolts after cold exposure or late spring frosts, so the pre-frost sowing window carries real bolting risk and the after-frost transplant window is the conservative option.
"Asian greens (like bok choi), and kohlrabi, are known to bolt prematurely if they are exposed to cooler temperatures, so they should be set out after last frost."