Marigold (flower) 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
Sources disagree on indoor timing - UW-Madison says about 8 weeks and UMN about 10 weeks before the after-frost outdoor planting date - and the later, more conservative 8-week start wins, giving 56 to 42 days before last frost.
Transplant outdoors
from the median last spring freeze to 2 weeks after
UMN, UW-Madison and Iowa State all agree that marigold plants go out once danger of frost has passed, so the set-out window opens at the last-frost date and runs two weeks.
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
Iowa State University Extension is the only one of the three sources to anchor direct seeding to frost rather than soil temperature ('Sow marigold seeds outdoors when the danger of frost is past'), giving a two-week window from the last-frost date; UW-Madison's and UMN's 65F soil-temperature rules cannot be frost-anchored and were not encoded.
Variety assumption: Covers Tagetes marigolds (French, African/American and signet types) only; pot marigold / calendula (Calendula officinalis) is a different plant and is not encoded here.
"Marigolds can be direct seeded into garden soil when soil temperatures are >65°F or seeds can be started indoors ~8 weeks prior to transplanting outdoors after the threat of frost has passed."