Ed, a diary is a method of taking notes at a location so that you know what was done, or any conversations that took place. For instance, if a customer tells you that he doesn't want you to clean the grease containment system on the roof, you enter it in your diary, and you have it tracked so that there is no legal repercussions. It is also helpful in larger companies to know what other people have told a customer. You will find that large companies such as insurance companies, and car manufacturers, or any company that runs the risk of multiple people talking to one customer will use a diary to track the conversations.
For what you are doing, I use google calendar. I am fairly certain that I have more people going more places then you do, and am able to use google calendar to keep track of them quite successfully.