RPGMemes

This depends a lot on interpretation and edition etc etc. If we're looking at 5e Forgotten Realms, historical inventions and their proliferation/use and the technology of the real world, then "D&D" is "set in the period" of about 1850.

Waterdeep (perhaps the most developed city on Toril) has movable type printing press. There are multiple newspapers in circulation with a similar level of proliferation as mid 1800s Birmingham (a city I selected for having a similar size and population). Waterdeep has widespread indoor pluming and sanitation, at least for rich people (the Water Closet was invented in 1777), It doesn't have the internal combustion engine (1872). This puts the "period" somewhere between 1820 and 1880. Mayonnaise is canon.

It can be very difficult to map "real history" onto fantasy settings with magic - necessity is the mother of invention, and many common problems that WE solved with invention would have been solved in such worlds through magical means. Why develop trains when you have teleport circle? Why develop the telephone when sending exists and you can make sending stones? Why develop penicillin when lesser restoration is a second level spell? - In general it's worth assuming that the lack of an invention in a fantasy world doesn't restrict the "time period" to before then.