The Clock by Christian Marclay is a montage of film clips that runs for 24 hours and is made up of time references for every minute of the day.
It's a collage of thousands of movie clips that makes a perpetual artwork.
"This is extraordinary, it's artwork about time, where you sit and think about time, and are reminded about time, all of the time." (Will Gompertz).
I think it's such a fascinating piece because it reflects one of the key modern truths about our society:
We are time obsessed.
Alain de Botton, as always, makes a brilliantly insightful comment on the modern tyranny of time:
"Modern capitalism guards time preciously because it knows it to be one of the raw ingredients of production and financial accumulation. Just as valuable in its own way as oil or a labour-force. As a result time has become oppresive."

