A full century is 100 years old. 100 years must therefore pass before a new century can begin. Since our era begins with the year 1 AD, the 1st century ended with the end of the year 100. Thus, the year 1900 had to end first before the 19th century could be finished. The 20th century also lasted 100 years and ended with the beginning of 2001.
The reasoning is the same for a millennium. A millennium lasts 1000 years. The second millennium began on 1 January 1001 and ended on 31 December 2000.
Each era begins with year 1. There is no such thing as year 0.
A brief historical overview
Among the Romans, who laid the foundations of our calendar, the era began with the foundation of Rome, fixed in 753 BC (Ab Urbe condita or AUC). However, there were other starting dates. Since the 3rd century AD, the years have been calculated from the ascent of the Emperor Diocletian. Since he persecuted Christians, this time was called the era of the martyrs.
In the 6th century, the monk Dionysius Exiguus (Dionysius the Little) began to count from the year that, according to him, was the year of Christ’s birth. Thus, 248 Anno Diocletiani was in fact the year 532 AD. According to Dionysius, Jesus Christ was born on December 25 of the year before year 1, he did not know the number zero, but there is no era in which a year 0 would be placed.
In the 8th century, the English Father Bede the Venerable also began to count the years before Christ. The year 1 BC was therefore just before the year 1 AD. The year 1 BC was therefore the last year of the 1st century BC.
It should be noted that this Christian way of counting years was used by the Church in the Middle Ages and much later in the civil world. There were many other ways to count. There were controversies about the date of the beginning and in particular about the year of Christ’s birth and Herod’s death in 4 BC, which should be brought forward by a few years, to around 6 or 7 BC.
You can find more information about calendars on The Calender FAQ web page.