Calendula (flower) 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
2 weeks before the median last spring freeze, up to the freeze date
Colorado State University Extension asterisks calendula in its seed-starting table to flag it as a plant best suited to direct seeding in the garden rather than an indoor start, and its footnote places that sowing two weeks prior to the average frost-free date, which UW-Madison corroborates by saying seed can be sown directly in the garden just before the last frost date, so the window runs from 14 days before the last-frost date to the last-frost date itself.
No start seeds indoors window: No indoor-start window is encoded because CSU's asterisk flags calendula as best suited to direct seeding rather than an indoor start, and that flag is followed here in preference to the 6-to-8-week indoor sowing UW-Madison mentions only as an option for earlier bloom.
No transplant outdoors window: No transplant window is encoded because neither source gives a frost-relative date for setting calendula plants out in the garden.
Variety assumption: Covers Calendula officinalis (pot marigold / English marigold), a hardy annual distinct from the Tagetes marigolds encoded under the separate 'marigold' entry; CSU's 4-week cold-frame and 4-week indoor columns for calendula are not encoded because the same row's asterisk marks the plant as better suited to direct seeding.
"Seeds can be sown directly in the garden just before last frost date, but for earlier bloom start seeds indoors about 6 to 8 weeks before last frost."