I used to use Evernote, since moved to OneNote. Just a page per day with a table (I have a template). One column is some amount of time (usually valued in pomodoros, sometimes minutes for meetings). The other column is just what I was doing during that time. The full-text search and sync is really the big advantage of using those note taking platforms. It’s weirdly wonderful to be able to grab a note from like 6 years ago and know exactly what I was working on that day.

I’m FTE now, but I’ve actually billed hours by totaling these logs in the past and it worked well.