AD stands for two Latin words, ‘Anno Domini’, meaning 'in the year of the Lord' (i.e. Christ) . The year 2007, also written as AD 2007, means 2007 years after the birth of Christ. BC stands for 'Before Christ', meaning before the birth of Christ. For example, a time period written as 200 BC means 200 years before the birth of Christ. BC runs into countdown format as 10..,9..,8..,7..,........1.. upto the time of Jesus Christ's birth and after his birth it started to run in today's date format, i.e., 1..,2..,3....,onwards.