If you were stranded on the desert island, what would you need to watch for?
Though you could just stick a piece of wood into the sand, mark shadows thrown at sunrise sunset and divide the circle's span however you want for your time system..
Without other people to coordinate with, the signals from the environment and your body are a more sound structure for these. The time of day is at best a good proxy.
as an approximate compass, if it has an hour hand (bisect the angle between the hour hand and noon, that's north or south, depending on your latitude, unless you're in the tropics where this doesn't work)
as a heliograph to flash signals to passing ships
if there's a date magnifier, you can use it to light fires
the engraving on the back may help identify your sun-bleached bones ("So, this poor devil was named WATER RESISTANT")
I've read and sort of understood and forgotten the technique before - but that's certainly not it: you could determine your north that way and spin around and it wouldn't change.