Hey grammar experts—

I have become confused about what dates the phrase “mid to late 1800s” best describes. (As used in “this event occurred in the mid to late 1800s”).

I always assumed that this phrase meant the event occurred from around 1850 to around 1890. But a friend of mine thought that this meant from around 1805 to 1809. (The hundreds (00s, 01, 02, etc) of the 19th C were key to him, rather than the century as a whole).

Which usage is correct?

Is the phrase at all ambiguous? Is it better to say “mid to late 19th C” if you mean 1850-1890? BTW, this originally concerned usage in verbal/oral speech, not writing, but I am interested if your answer would differ for usage in writing as well.