XIRR - Excel Internal rate of return for scheduled cash flows

One function that I had dearly wanted was to be able to calculate IRR for cash flows on different days, rather than just in yearly intervals. The good news is that Excel provides this functionality already. According to this page :

Returns the internal rate of return for a schedule of cash flows that is not necessarily periodic. To calculate the internal rate of return for a series of periodic cash flows, use the IRR function.
If this function is not available, and returns the #NAME? error, install and load the Analysis ToolPak add-in.
How?

1. On the Tools menu, click Add-Ins.
2. In the Add-Ins available list, select the Analysis ToolPak box, and then click OK.
3. If necessary, follow the instructions in the setup program.

Syntax
XIRR(values,dates,guess)