Needless to say, I still have the option of pushing a formula into a
cell, pushing the arguments into other cells, excuting the formula
cell, and then picking off the result.

When I go that way, however, it still throws an error.

I come up with a FormulaR1C1 of something like:
=MDURATION("1/1/2008", "1/1/2016", ".08", ".09", "2", "1")

And, indeed, when I manually paste that into a spreadsheet that I have
created by opening up Excel manually, I get the expected result.

But when I programatically put it into a spreadsheet I have opened via
code, it gives an "Error 2029".

If I copy the exact formula the code has created from the cell it was
created into and paste that into another sheet, it works too.

Can anybody elucidate?