Has anyone noticed substantial calculation errors in EXCEL 2008 - specifically, the XIRR and XNPV function? I ran a cash-flow series and then ran both the XNPV and XIRR function. Then i used an alternative calculation methodology, using the SUMPRODUCT and Goal Seek functions, to calculation the NPV and IRR of the same series of monthly cash flows (note that these alternative calculation methodologies are used to produce the same incremental calculations that are supposed to be rolled up into the XNPV and XIRR functions). Results: the XNPV is off by 1.26% and XIRR is off 26.77% I guess that isnt to bad when considering the investment of millions of $. XNPV would only cost me $126,000 on the million. So much for Gold Standard of financial analysis. What ever happened to Lotus 1-2-3? It might be archaic but at least it generated proper answers.