Clock tower of Emmanuel College, University of Cambridge, England (© Geography Photos/Universal Images Group/Getty Images)

In the quadrangle of Emmanuel College, Cambridge, the clock tower stands as a silent witness to the passing hours. But on the last Sunday in October, it is given a peculiar job: striking time twice. At 2 a.m., British Summer Time (BST) officially ends, and the United Kingdom reverts to Greenwich Mean Time. One moment it's 2 a.m., the next it's 1 a.m. again.