Errata ....

On Oct 22, 9:39*pm, I wrote:
1. Do you know that for XNPV, "rate" is the annual
discount, but for NPV, "rate" is the periodic
discount rate?

But you wrote: "surely if the dates are periodic
(in this case annually over 12 years) it should give
the same result as the standard NPV calculation?"

So this speculation does not apply to you.

However, my speculation about the exact daily computation of XNPV
might. Over 12 years, there might be 3 leap years. But again, the
difference should be small. And again, a numerical example would
improve your chances of getting a dispositive answer.