Clocks are an essential part of society, hanging in most homes, workplaces, in our pockets, and around our wrists. This makes clocks a prime target for art, and they happen to be a specialty of Dutch artist Maarten Baas.

The Sweepers Clock is a top-down 12-hour recording of two men in overalls walking around in a circular area, sweeping a pile of trash into two lines that come to be the clock’s hands. They move the trash each minute in order to adjust the clock hands to give the right time. The clock hangs in the Erasmus University Medical Center, and you can see a video of it in action here.

This clock is part of Baas’s “Real Time” series. It includes three other clocks, one of which hangs in Schiphol Airport and features a man in blue overalls painting the clock hands each minute.