Well, IMHO, this is getting more complicated than it needed to be.
I really don’t like XIRR. I don’t like it because it uses the *TIME* instead of the cash flow to ponder the profits. And what an investor should be interested in should be the cash in cash returns (or the simple IRR). (Of course, you would like to estimate your annual gain, after, say, a 5 years period, something like 20% by year - to compare to an index, but I mean past periods, not future….)
The XIRR will *estimate* the return over one year, even if you only get one month return.
That will looks great for making everything you want with numbers. (Or pretend you are a great investor counting on your annual return in a bull month and than comparing it to the index. – In fact, I have seen more people that don’t know how to measure their performance saying too much and proving too little….)
In my investing word I would prefer something like that:
2007 - 20%
2008 -25%
2009 -13%
2010 - Jan – 3%
Since beginning - 75%
Medium 19% per year
Let’s get the example in spreadsheet in the article you mention:
Date Return (Payment)
01/01/2006 $ (100.000,00)
21/09/2006 $ 120.000,00
XIRR = 28,8%
As you can see, the return the XIRR give to you is 28,8%, as an estimate of the annual return. But, in fact what you deserve to know is that your actual return that is: 20%. That is what is your real return is at that point! That is also your base for collecting taxes. What WILL BE your return at the end of the year no one knows.
When you finish the year take your time to make the proper comparisons
But, any way, after you organize the data, the dividends, the deposits and the withdrawn of the money the mathematics of the TIR or the XTIR is just a matter of a formula.
As far as I know, the article I mention in my previous post is the way people in the fund business use to calculate their NAV.
Best regards