"A furniture item maded in the year 1900 is 19th century, maded in 1800 is 18th century" Since when?

"When does the 21st century begin?" The first century began at the beginning of the first year. The second year began after the end of the first year. 100 years must pass before a century is complete. The second century began after the end of the first 100 years. Not at the beginning of the 100th year. At the beginning of the 101st year.

What year was the first year?