Archaeologists have found a stone tool assumed to be an early calendar dating back 4,000 years in a cave in the northern province of Tuyen Quang. According to Prof. Trinh Nang Chung from the Vietnam Archaeology Institute, the stone tool, with 23 parallel carved lines, seemed to be a counting instrument involving the lunar calendar. A similar tool was found in Na Cooc Cave in the northern province of Thai Nguyen's Phu Luong District in 1985, Chung said. Similar items have been found in various areas in the world, including China, Israel and the UK, suggesting that people 5,000 years...