BC to AD Calculator
This calculator converts years between the BC (Before Christ) and AD (Anno Domini) dating systems and works out how many years have passed between two historical dates, even when one falls before Christ and the other after. Enter a start year and select whether it falls in the BC or AD era, then do the same for an end year; both must be positive whole numbers. The tool returns the elapsed years between the two dates as its headline result, plus each year's equivalent in astronomical year numbering, the system astronomers use where 1 BC becomes year 0, 2 BC becomes -1, and so on. Below the results sits a detail table showing both years in historical and astronomical form, plus a breakdown showing how the astronomical years were subtracted to reach the final answer. This matters because the BC/AD calendar has no year zero: 1 BC is followed directly by 1 AD, so adding the two year numbers alone gives the wrong answer. The correct shortcut is BC year plus AD year minus 1, which the tool applies automatically. Use it to work out how long ago an ancient event occurred, how many years separate two historical dates, or to convert a BC year from a book into astronomical notation. BC and BCE mean the same thing, as do AD and CE, so use either labelling interchangeably.
