The open-source ai-bookkeeper sales tax return assumes the 20th of the month after the period unless a state is listed otherwise. We added three states that differ:
- Maine: the 15th (Maine Revenue Services Rule 304), moved to the next business day on a weekend or holiday.
- Vermont: the 25th (Department of Taxes FAQ). Monthly returns for January are due the 23rd; that exception is not modelled yet.
- North Dakota: the last day of the month after the period (third-party guides; the state site was not read).
Iowa stays out. Quarterly and most monthly returns look like month end, but monthly prepayments are the 20th and guides conflict, so a single number would mislead.
These come from search excerpts, not the full state forms. Check your own notice before filing. This is an estimate tool, not tax advice.