The Gregorian and Julian calendars, which are widely used to count historical years, do not include a year zero. Instead, the sequence goes from 1 BCE (Before Common Era) directly to 1 CE (Common Era). This dating system is standard in historical and scholarly works, including those published by Britannica.
This absence of a year zero can affect calculations involving dates spanning BCE and CE, such as determining the number of years between two historical events. When calculating across the BCE/CE divide, it's important to account for the missing year zero by subtracting one year from the total span.
The use of no year zero is the result of conventions established by early medieval scholars, and it remains the norm in most historical timelines and references today. Alternative systems, like astronomical year numbering, do include a year zero, but this is not the case for the commonly used Gregorian or Julian calendars.