A single measured task is worth more than a dozen remembered ones. The point is not to log your day, it is to find out where your estimate goes wrong.
Start it when you begin a task, lap it at each natural boundary — each page, each email, each build — and stop when you are done. The lap list is the record; the summary shows the fastest, slowest and average segment.
Because most people are systematically wrong in the same direction. We estimate the work and forget the setup, the interruptions and the tidying up, which is why everything takes longer than planned. Timing a task once replaces the guess with a number — and the largest time-waster is usually the one nobody put in the estimate at all.
Anything you would want to see broken out later. If a task has a slow part that keeps annoying you, lap on either side of it; the segment times will show how much of the total it actually eats, which is often less than it feels.
No. The lap list lives on the page and disappears on reload. That is deliberate — it is a measuring tool, not a timesheet. Write down the number that matters before you close the tab.