I am aware of methods to make the visibility of scenery dependent upon seasons, time of day, and specific dates. The method involves creating different textures for the scenery object, and setting the season/time of day/etc. in the ini file when running resample to compile the textures. I've done this to create seasonal and day/night ground textures for airports.
Unfortunately, I do not think that the method would be suitable for setting the dependency upon a specific date, with the object being not visible before the date, and visible on or after.
The easiest solution I can think of is to create a stand alone scenery BGL, and add it as a library which can be turned on or off easily with your preference of what era you choose to simulate.