[WilliamMayor]

I really like using a calendar for this. I use Google Calendar and iCloud in Sunrise, on my MacBook, iPhone, and iPad.

Events get a title that provide the project. They then get more detailed in the description. I also have a Google Apps Script that pulls this information into a spreadsheet so it's easy for me to report on my time.

What I like about this system is that it's pretty universal, fits into my current workflow, and it's flexible. Every platform has a calendar app, I don't have to do anything fancy to get my time tracker up and running on a new device. I'm already using my calendar to tell me what things I'm doing today, now I'm also using it to tell me what I did yesterday. Changing dates, times, and descriptions is easy, you just edit a calendar event, you can even do it on the train home when you realise that you've forgotten to update.