The 20th century began on January 1, 1901, and ended on December 31, 2000. This time frame is considered the 100-year period of the 1900s, which was marked by significant events in history, culture, science, and technology.
The confusion often arises because some people mistakenly consider the century to start in the year 1900. However, centuries are defined by their full range, meaning that the first century included the years 1 to 100, the second century ran from 101 to 200, and so forth. Therefore, the 20th century properly spans from 1901 to 2000.