ai-bookkeeper's salestax return worksheet gave North Carolina the 20th for every filing frequency, taken from third-party guides. NCDOR's "Filing Frequency and Due Dates" page says otherwise: monthly filers file by the 20th, but quarterly filers by the last day of January, April, July and October. A Q1 return was shown due April 20 when it is due April 30.
stateDueDay("NC", ...) now returns month end for non-monthly periods, with a test for each case. Georgia's 20th was confirmed on dor.georgia.gov the same way, and Colorado and Minnesota's 20th agree with third-party tables (their own pages were not opened). Other states still rest on search excerpts; the README lists which.