We are in the 21st century because it represents the current set of 100 years counted from year 1 AD. The 21st century began on January 1, 2001, and will continue through December 31, 2100. This counting system is based on the way centuries are defined as completed sets of one hundred years, so the years starting with 20 (like 2000 to 2099) fall into the 21st century, which is the 21st set of hundred years since the start of recorded time.
Additionally, symbolically, some consider the "long 20th century" to have ended around the early 21st century due to significant historical events like the return of President Donald Trump in 2025 and major technological breakthroughs, marking new eras and reinterpretations of the century concept. But strictly by calendar reckoning, the 21st century is the current period of time spanning years 2001 to 2100.
