It does not know how long it runs. Flip it a few times and it will find out.
Changing any of these empties the glass and starts a new page in the ledger. The sand itself is not adjustable, because sand is not adjustable.
Flip it once and it will start keeping a record.
Every grain in there is a real body. The sand is a few hundred small circles with friction and almost no bounce, falling under gravity through a narrow neck — not a fill level going down, and not an animation on a timer. That is why the pile heaps at its own angle, why the flow stutters when the neck is tight, and why the last of it drains slower than the first as the heap above the hole runs out of head.
The clock is real and the glass is not calibrated. A real hourglass is filled and ground and tested until it runs the span it is sold as. This one is not: it runs for however long the physics takes. So the page does the only honest thing available — it times each run with the same clock your computer uses, writes it down, and tells you what to expect from the runs it has actually seen. Six flips at one setting and it becomes a usable egg timer.
Flipping rotates gravity with the glass rather than teleporting the sand, so the pile tumbles the way a real one does when you turn it over — and yes, you can flip it mid-run and watch it change its mind.
Times are measured on your own machine and stored only in this browser. A tab left in the background is not being timed — a run paused that way is discarded rather than written down as a slow one.