ai-bookkeeper's salestax return worksheet keeps a table of due days by state. Many entries came from third-party guides. Checking Colorado and Minnesota against the states' own pages (through search excerpts, not the whole pages) turned up one error: Minnesota annual filers, those averaging under $100 a month in tax, are due February 5 of the following year, not January 20.
Colorado held up: the 20th for monthly and quarterly, January 20 for annual.
The fix needed a new kind of due day, "the Nth month after the period, on day D", because the old model only knew a day in the month right after the period. One test pins both states. The full suite (815 tests) passes.
The worksheet still tells users to check their own notice. States change schedules, and an excerpt is not the form.