Getting this achievement is quite easy, but hidden away.

Step 1 - Load game and go into map editor.
Step 2 - On the right hand side of the screen toward the top, you will see the option for "custom" and a greyed out settings box, click on custom and you will be able to enter settings.
Step 3 - In the settings section for this map, you will see "mods" listed below, if you haven't loaded any extra's there should be only three, select "sandbox mode" (it will turn green showing it's active)
Step 4 - Back out of the settings and continue with generating the map, once you've got your blank canvas open, feel free to make your own map, or randomly generate one.
Step 5 - Before exiting change the date to something like 1950 (pick whatever you like)
Step 6 - Once you've finished creating your map, save the map.
Step 7 - Go back to the main menu and start a "Free Game", you will see in the top right corner "Play Map", select the map you just saved and start the game
Step 8 - Last but not least, click on the date, in sandbox mode, you will now have the option to type in the date you want, for example 1850, hit enter.
Step 9 - Profit