Null test:
{ Flow
Sum{  = 0
{ (1+r)^t
Typical spreadsheet cell:
Flow*(1+rGuess)^(tFlow/tBase)
Observe sign of flows

Discounted cash flow [DCF] is compound interest with
time's arrow running backwards.
Each flow is discounted by time, from the flow's date
back to the initialization date (today). This is done by
iterating a guessrate into a discount formula stack, one formula
for each flow, until the null value is zero [0].
DCF can be done in any
spreadsheet for any timebase  and can be more accurate than XIRR.
XIRR is easier but less intuitive.
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