Sandaq, why use 31/12/2007 ? we're actually 16/05/2007, and this should be your last date. Here is a link to an example, with spreadsheets to download, etc. http://www.fatpitchfinancials.com/392/how-to-calculate-your-return-on-investment/ Hope this will help, Olivier.
And by curiosity I tried your numbers (with a little modification : I use the 16 of may as last date, instead of the 15, to have at least three dates (otherwise were just deleting the last addition, and computing a basic coumpounded RoR), the results are : Money weighted return (XIRR) : 8.71 % <=> 662.24% annualized ; Time weighted return (Modified Dietz, EOD calculation) : 8.69% <=> 660.73% annualized. Olivier.