How to Calculate Years Across BC and AD

By Michael O. Smathers; Updated April 24, 2017

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.

About the Author

Michael Smathers studies history at the University of West Georgia. He has written freelance online for three years, and has been a Demand Studios writer since April 2009. Michael has written content on health, fitness, the physical sciences and martial arts. He has also written product reviews and help articles for video games on BrightHub, and martial arts-related articles on Associated Content.