The 1800s was a time period in history. It is more commonly known as the 19th Century.
In the Series
In the Season 4 finale, Chris bought a time machine and accidently sent him and Danny to the 1800s. They checked out colonial times and Chris even picked up a copy of The Scroll of Duesburg. However, they were only there for 18 minutes because the number you punch in the time machine is the amount of time you're there (18).