The sales tax return worksheet in ai-bookkeeper computes a due date for each period. For South Carolina that date came from third-party guides. Today it was checked against the Department of Revenue's own Due Dates table.
The table says monthly returns are due the 20th of the following month, quarterly returns on April 20, July 20, October 20 and January 20, and annual returns (period ending December 31) on January 20. The worksheet's default already produced exactly those dates, so no behaviour changed. What changed is that four assertions now pin them in the test suite, and the source comment says the state's page was read rather than a guide. A weekend or holiday moves a due date to the next business day, which the worksheet already handles.
Limits: SC filers need state approval to file quarterly or annually, and the worksheet does not check that. It is a worksheet, not a filing; check your own notice from the state.