XIRR(values,dates,guess)
values: cash flow
dates: dates of cash flow
guess: an estimate of what the result should be
XIRR calculates the annualized internal rate of return of a cash flow at arbitrary points in time. values lists the payments (negative values) and receipts (positive values) with one value for each entry in dates.
The optional guess is needed because there can be more than one valid result. It defaults to 10%.
IRR.