> Set a timer to go off every 15 minutes
I realise this won't be a popular point of view with the folk who are always 'life-hacking' or what not, but my goodness that sounds mortifying to me.
I have worked in professional services too, and I had to bill for my time in 6 minute intervals, but I found it much easier to just switch between set of running timers when I changed tasks. No need to put together any timesheets at the end of the day, just note/submit the final times. I think a repeating 15 minute timer would literally cause me to go insane.
I'm all for people staying on top of their schedules, but I doubt that for most people the benefits of something like recurrent 15 minute timers could ever outweigh the costs (concentration, flow, mental health).
I use an Apple Watch, and it's a gentle tap on the wrist that's perceptible to me and basically no one else.
When I say "write everything down" I mean use the lightest of note taking. It can be a bullet point for the day, or an expansive note on a particularly difficult technical challenge, there is no one set way of approaching this.
The timer is pretty unobtrusive. It goes off, I write one or two words in a box, and I carry on.
The daily update in the spreadsheet is extremely useful though.
I'm low tech and prefer to use a notecard with a grid on it, the squares of which bear enough space for only a word or two maximum, but you can just as easily use a spreadsheet or something fancier.
It makes putting together a better summary at the end of your day a lot easier, and it also helps with future forays into time estimation since you have a clearer picture of how long things take.
It also helps show when it's time to go take a walk.