Kohlrabi (cool-season stem brassica) 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
Start seeds indoors
10–8 weeks before the median last spring freeze
Sow 4–6 weeks before the set-out window opens (UNH: time from seed to transplant 4–6 weeks).
Transplant outdoors
4–2 weeks before the median last spring freeze
UNH: safe to set out 4 weeks before the last frost; Purdue's hardy class allows 4–6 weeks before. The later UNH start is used.
Fall planting
8–4 weeks before the median first fall freeze
Purdue Table 2: plant fall kohlrabi at least 4–8 weeks before the first fall frost.
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.