Many many thanks for all your replies.
I posted on the data, dates and numbers I used as well as the results....

The NPV difference is huge .....

And it doesnt seem to hold to add the first year/numebr/data point back in....

I really am stumped!!

"joeu2004" wrote:

Errata ....

On Oct 22, 9:39 pm, I wrote:
On Oct 22, 8:47 pm, Energy wrote:
Why do you get different NPV values using NPV
and XNPV, if the timeframes are equal distance
for the XNPV calculation?
[....]
Do either or both of these factors explain the
difference you are seeing?
If not, again, please post numerical details.

Following my own advice, I tried the following:

=npv(10%,A1:A3)
=xnpv(10%,A1:A3,B1:B3)

where A1:A3 contains -10000, 3000, 4200, and B1:B3 contains 1/1/2009,
1/1/2010, 1/1/2011. Note that those 1-year periods are exactly equal
in length (365 days).

NPV results in about -3456.05. XNPV results in about -3801.65.

Neither of my previous speculations would explain such a large
difference.

The explanation is: Excel's NPV discounts the first cash flow,
whereas XNPV does not.

Note that the following matches the XNPV results:

=A1 + npv(10%,A2:A3)