Sweet Marjoram (herb) is classed 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
8–6 weeks before the median last spring freeze
NC State's Extension Gardener Plant Toolbox starts marjoram seed indoors about 6 to 8 weeks before the last spring frost date, and Clemson HGIC gives the identical 6-to-8-week lead, so the two sources agree.
Transplant outdoors
from the median last spring freeze to 2 weeks after
Clemson HGIC sets marjoram transplants out after the last killing frost and UMD Extension names marjoram among herbs planted outside after the danger of frost has passed; both agree, so the window opens at the last spring freeze.
Direct-sow outdoors
2 weeks before the median last spring freeze, up to the freeze date
NC State's Extension Gardener Plant Toolbox gives a point rule — sow seed directly outdoors about two weeks before the last spring frost date — which opens a two-week sowing span ending at the last freeze; note the transplant window for this tender herb is later.
Variety assumption: Grown as an annual across most of the US; NC State's plant toolbox treats it as a tender perennial that is replaced each year in cold climates.