Human civilization has lasted more than 7,000 years. The year-numbering system is centered on the birth of Jesus Christ, with "B.C." marking the number of years prior to his birth. Occasionally, during the study of history, you will have to calculate the amount of time that passed between two dates on different sides of the divider.
Determine the earlier date during the B.C. era. For example, if you want to find out the time interval between the Battle of Thermopylae and present day, you would use 480 B.C. as your earlier date.
Determine the latter date on the A.D. side.
Add the two numbers. For example, if you want to use 2011 as the present year and determine how long ago Thermopylae took place, add it to 480 to get 2,491. The Battle of Thermopylae would have taken place 2,491 years ago as of 2011.