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Is There a Year Zero?
How BC transitions to AD
The world counts years with the Anno Domini (AD) system in which the current era is counted from the birth of Jesus Christ. Scholars and secular students have started using Common Era (CE) to replace AD but the numbering is exactly the same. Prior to AD we have Before Christ (BC) or Before Common Era (BCE) which denotes the years leading up to the birth of Christ.
This system begs the question about a year zero. How do we transition from BC to AD?
Year zero AD
At first glance, it would make sense for there to be a year zero in our calendars. The numeral system includes zero. That zero divides negative numbers from positive numbers. Despite that seeming similarity there is no year zero in the Anno Domini system of numbering the years.
The years from the former era and current era simply goes from one to one. The last year of the BC era is 1 BC and the first year of the current era is AD 1. There is no year zero.
There is some debate about whether or not Jesus was born in the year 1 BC or the year AD 1 but his birth is what divides our timeline into the current epochs that we are so familiar with. Despite the debate, everyone jumps from 1 BC to AD 1 with no non-value year in between.
Other year zeroes
While the most widely accepted calendars do not use a year zero, there are some numbering systems that do. The most prevalent is the astronomical year system which sets all of the years to simple integers. The astronomical year system lines up almost exactly with the Anno Domini system except the astronomical years use a year zero to denote the demarcation between BC and AD.
They changed their system so that the year 1 BC is now the year 0. Years preceding year zero are numbered -1, -2, -3 and so on while years following the year zero go up as 1, 2, 3 etc. This system is used mostly in astronomy where it is easier to use a simple integer line to graph dates rather than using the current system.
Other lunar, religious and cultural calendars also use year zeros to mark special occasions or the differentiation of eras but those are largely regional and local traditions.
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