Rhubarb (perennial vegetable) is classed perennial 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
Transplant outdoors
6–4 weeks before the median last spring freeze
Purdue Extension HO-186-W Table 1 lists rhubarb in the group headed "Hardy (tolerates hard frost)" with the instruction "plant 4-6 weeks before last spring frost (see Figure 1)" and footnotes it as typically vegetatively propagated, so the source's own 4-6 week range is used directly as the window for setting out dormant crowns rather than seed; the University of Minnesota rhubarb guide was re-checked and anchors only indoor seed starting to the last frost, so it supplied no competing crown-planting date to reconcile against.
No start seeds indoors window: Rhubarb is planted from crowns or divisions, and although the Minnesota guide does say to start seed indoors six weeks before the last frost, seed-grown plants push the first harvest to the third season and are not the planting path this window times.
No direct-sow outdoors window: Purdue Table 1 footnotes rhubarb as typically vegetatively propagated and publishes no separate direct-seeding date for it.
Variety assumption: A long-lived perennial planted from crowns or divisions: Minnesota advises waiting until the second season before harvesting (the third season if started from seed), with the harvest season running until the end of June.