"What have you been doing in the last 15 minutes?" - "I've been trying to get back into the task I was working on after my concentration was broken because I needed to update this time tracker"

Nail on the head

I briefly worked for a law firm and had to track my time in 6 minute increments.

Could you work on one thing for an hour then back-fill the timesheet to reflect that?

Yes, of course, but it became quite difficult to track when there was fire fighting across several fires