"The 1800s" typically refers to the years 1800-1809. "The 1900s" refers to 1900-1909. It is by no means unambiguous, but this is the common and plain meaning, and indeed these folks may confuse many more people by attempting in this manner to refer to 1800-1899 or such.
"The 1800s" typically refers to the years 1800-1809. "The 1900s" refers to 1900-1909. It is by no means unambiguous, but this is the common and plain meaning, and indeed these folks may confuse many more people by attempting in this manner to refer to 1800-1899 or such.