Is there something about the Help description for XNPV that is not clear?
XNPV "Returns the net present value for a schedule of cash flows that is not
necessarily periodic. To calculate the net present value for a series of
cash flows that is periodic, use the NPV function."
For XNPV you must specify dates for the cash flows. For NPV the cash flows
are assumed to be equally-spaced time intervals.