January 1, 1901 - December 31, 2000 is the definition of any day that is in the 20th century January 1, 2001 - December 31, 2100 is the definition of any day that is in the 21st century Therefore, today is the 21st century. *see comments below

Short answer, yes Start at beginning which is Year 1. Considered 1 AD. (anno domino, latin for year of our Lord, now sometimes seen as CE as common era) Year 1-99 or 100. That is the 1st century. 2nd century is 100 (or 101) to 199. 3rd century is 200-299. Etc etc So the 1900’s are (is?) the 20th century Using this India-aramic system of numberng, a person in the 3rd decade of life is actually in the 20’s. Hope this helps. Paul