BCE and CE, why there is no year zero, how to count centuries, sexagenary years and reign years, and lunar dates, explained without confusion.
A timeline lists events in order, but read well it tells you three things. First, sequence: what came before and after. Second, intervals: how many years separate two events changes how you view cause and effect. Third, simultaneity: what was happening in different regions at around the same time. Textbook timelines often space events evenly, which can hide the real distance in time, so it is worth checking first whether the scale of the axis is consistent.
The common era system used widely today is divided into BCE and CE. For BCE dates, a larger number means further back: 3200 BCE is long before 108 BCE. Many writers use BCE and CE instead of BC and AD to drop the religious reference, but the numbers are the same. The key point is that there is no year zero. The year immediately after 1 BCE is 1 CE, so when calculating a span that crosses from BCE into CE, add the two numbers and subtract one. For example, from 108 BCE to 918 CE is 1,025 years.
A century is a block of 100 years, but because counting starts from year 1, the year ending in 00 is the last year of its century. Once you know this principle, the arithmetic is simple.
Timelines and books often use approximate expressions instead of exact years. The 1990s means 1990 to 1999. The first and second half of a century usually mean the first and last 50 years, while early, mid and late refer roughly to thirds. Words like around, circa and estimated signal that the sources are too thin to fix an exact year. Turning such expressions into precise numbers creates certainty that was never there, so when quoting, keep the original wording.
Old records did not use the common era. Joseon records mainly dated events by the king's reign year, so the creation of the Korean alphabet is recorded as the 25th year of Sejong, that is, 1443. The sexagenary cycle, which repeats every 60 years, was also widely used: the war that began in the imjin year of 1592 is called the Imjin War, and the war of the byeongja year of 1636 is called the Byeongja Horan. The Dangun era is the common era year plus 2333, and it was the official year system in the Republic of Korea until 1961. Japan still uses era names alongside the common era.
Most dates in Joseon records follow the lunar calendar. Korea officially adopted the solar calendar on 1 January 1896, which was the 17th day of the 11th lunar month of 1895. Comparing an old record's date directly with today's calendar can therefore be off by close to a month. Korea's Hangeul Day on 9 October was likewise set by converting the early ninth lunar month recorded in the original Hunminjeongeum text into the solar calendar. In the West, too, the Gregorian calendar was introduced in 1582 but adopted at different times by different countries, creating the same problem.
Drawing a timeline yourself builds a sense of time far faster than reading someone else's. Try making a one-page timeline with the method below. Checking the number of years between two points with an age or date calculator reduces mistakes.
The most common mistakes in date arithmetic are counting a year zero, which makes spans across BCE and CE one year too long, and treating the 1900s as identical to the 20th century. The year 1900 is the last year of the 19th century, and 2000 belongs to the 20th. People also often mistake lunar dates for solar ones, or forget that the sexagenary cycle repeats every 60 years and confuse years with the same name. When reading foreign books that write centuries in Roman numerals, a Roman numeral converter helps.
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