Some cells divide in correspondence with a circadian rhythm.
Today’s invention is a wristwatch which makes use of this fact. A transparent duct containing suitable cell food substrate on the back face is formed into a spiral. This is lit from beneath to enable photosynthesis in eg cyanobacteria.
A small number of bacterial cells are located at the centre of the spiral. As the bacteria divide throughout the day, and feed on the substrate, they expand along the spiral in a deterministic way, passing gradations marked on the duct wall which indicate the time. The marks could even be spaced linearly by varying locally the width of the spiral track.
The spiral becomes filled over a 24 hour period…at which point, a small defined area of the bug population is scraped into the start section of a fresh spiral and the rest (carefully) discarded.