British Summer Time (BST) begins on the last Sunday of March, when clocks are set forward by one hour, and reverts to Greenwich Mean Time on the last Sunday of October. The practice aims to provide more daylight in the evening and less in the morning, a concept initially proposed in the early 20th century by William Willet. He argued for the change to extend daylight hours for recreation and to reduce lighting costs. The idea gained traction during World War I when Germany adopted it to save energy, leading Britain to follow suit shortly after. The phrase "Spring forward, fall back" is commonly associated with the time change, highlighting the seasonal adjustment. BST is primarily observed in the United Kingdom.