Calculating Returns

I have a portfolio of mutual funds and stocks. Over the years money
has gone into this account, and money has come out. By this I mean I
take money away from the account and I send new money into it. These
movements are random, that is they are not regular, monthly or
anything like that. How do I calculate the return? I have heard of
irregular rate of return. Is this what I should use? Does excel have
this? Any advice?