Sunflower (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
3–2 weeks before the median last spring freeze
Illinois Extension's '2-3 weeks before planting outdoors' is measured from the after-frost outdoor sowing date, which places the indoor window at 21 to 14 days before last frost.
Transplant outdoors
from the median last spring freeze to 2 weeks after
NC Cooperative Extension (Pender County) names sunflowers among the large-seeded plants to transplant outdoors after the threat of frost, giving a two-week set-out window from the last-frost date.
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
Illinois Extension and the NC State Extension Gardener Plant Toolbox both direct-seed sunflowers after the last spring frost, giving a two-week window from the last-frost date; UGA's 'several weeks after the last frost' is later but not a number, so it could not be encoded and only argues for a later start within or beyond this window.
Variety assumption: Assumes annual garden and cut-flower sunflowers (Helianthus annuus); the indoor window is deliberately short because sunflowers resent root disturbance.
"Sunflowers are typically direct-seeded after the danger of frost has passed (early to mid-May). They can also be started indoors, typically 2-3 weeks before planting outdoors, if you want to get a head start."
"Large seeded flowers and vegetables are the easiest to grow and include cucumbers, zucchini, squash, melons, cosmos, marigolds, nasturtium, zinnias, and sunflowers. To get a head start on the season, seeds of these varieties can be sown indoors in mid-March to transplant outdoors after the threat of frost."