Coding for Fun and Profit
I spent most of the day working on a very useful, reusable chunk of PHP code. I'll be able to use it in many current, future and even past projects, so I developed it off-the-clock. It is not fair to charge one client for code I will directly use in others' projects. It's also not fair to spend extra time on something that doesn't directly benefit the client.
So I spent a day off-the-clock, writing code to save me on-the-clock time. It just seems silly.