Dionysius Exiguus was a 6th-century monk and scholar known for developing the Anno Domini (AD) dating system. He was born in Scythia Minor, which is present-day Romania, and later moved to Rome. His work aimed to create a Christian calendar that would help determine the date of Easter. In 525 AD, Dionysius introduced his system in a table that calculated years based on the birth of Jesus Christ. His method gained popularity and eventually replaced the Roman calendar, becoming the standard in much of the Christian world. His contributions significantly influenced how we measure and understand time today.