The Short Answer
The 20th century was from January 1, 1901 to December 31, 2000. That is it. It sounds like a simple fact, but you would be surprised how often people get this wrong in professional settings. I have seen project timelines, academic papers, and legal documents mess this up, sometimes with real consequences. The confusion exists because the Gregorian calendar has no year zero. It goes from 1 BC directly to AD 1. Centuries are counted from year 1, not year 0. So the first century covers years 1 through 100, the second covers 101 through 200, and so on. By that logic, the 20th century covers 1901 through 2000. The 21st century started on January 1, 2001, not 2000, even though most of the world celebrated it a year early. I remember watching news coverage in 1999 and seeing anchors casually refer to it as the start of a new millennium. It was technically still the 20th century and the 20th millennium. In practice, how you treat this depends entirely on what you are working with. If you are writing a history paper, a legal document, or anything where precision matters, the technically correct dates are January 1, 1901 to December 31, 2000. If you are doing something casual like dividing up decades for a presentation, people will often accept 1900 to 1999 because it lines up cleaner with how most folks talk about "the 1900s." But if someone asks you directly, the accurate answer is 1901–2000.
One edge case I ran into recently involved a database migration for a client who stored all their date ranges in a system that auto-calculated century membership. The query was supposed to pull all records from the 20th century, but the developer had written the filter using integer division on the year field, which treated 1900 to 1999 as the 20th century. The result set was missing every record dated in 2000 and included a handful of stray entries from 1800 that slipped through due to malformed data. I ended up rewriting the entire filter to use an explicit BETWEEN clause. It took about twenty minutes to fix and saved them from a much larger problem down the line. Another thing people tend to miss is how this interacts with ISO 8601 date formatting. The standard represents years as four digits, which works fine for most of the century. But when you cross into the year 2000, some older systems that were built with two-digit year handling misinterpret 00 as 1900 instead of 2000. That is the Y2K problem in a nutshell. Even now, legacy systems with bad date logic can still produce incorrect century classifications when processing historical data. If you are building anything that deals with historical records, always validate your date parsing logic against known boundary cases like 1900, 1901, 2000, and 2001. The takeaway is that the technically correct answer to when was the 20th century is 1901 through 2000, and you should use that whenever accuracy matters. For casual use, 1900 to 1999 is widely understood but technically wrong. The gap between the two is small, but in the right context it is the difference between being precise and being off by a full year.
