Build 5.0.0.65 of xlsgen introduces a formula hinter, meant to ease the use of formulas in cells. At the workbook level, calling IsFormulaADate(formula), along with a formula, returns whether or not the formula returns a date. This is handy if you'd like to know beforehand the formula is going to need a specific cell number format, and act accordingly.

interface IXlsWorkbook, one new function
    IsFormulaADate([in]BSTR formula, [out, retval]BOOL* result);