I keep a text file open in my text editor and set up a macro to expand `tt` to the current time, so I end up with entries like:
02:52 AM - 03:24 AM Plan
03:25 AM - 03:37 AM ProjectX
04:16 AM - 05:17 AM ProjectX P
At the start of a day, I rename the file to the day's date, eg. 20170605.md and archive it. I can write scripts to figure out things like how much time I have spent on a project over the month, years, etc. Also things like how much time was productive (the trailing P).