Look into “unreal data layers”:
Then look into “widget slider”:
Then make it all pretty, and share results here 
and best way to connect it aall together:
- make umg widget with slider.
- use “middle man” setup for communication: make middle man blueprint with event for eg in game mode.
- make event in game mode: “EVENT_change_epoch”
- make event dispatcher in game mode: “DISPATCHER_epoch_changed”
- when you move slider call that event in game mode to change epoch
- when it is called trigger (start) dispatcher epoch changed
- both event and disspatcher should pass some variable that gives what is new epoch (best would be custom enum you declare)
- create all buildings/chunks as "compacted level: (not sure this name). Compacted levels are basically blueprints with meshes as objects
- place those bluidings/chunks on appropriate data layer (for epochs)
- assign to dispatcher epoch changed, anything you need to react to that
- create data layer changing event in game mode and also assign it to that dispatcher, this will change data layer and show/hide correct epoch chunks
- add more things like lights sky, weather to that epoch changing (must be blueprint that assigns to dispatcher, just like game mode)
ps.
take a look into “prefabricator” plugin on FAB.COM, you may want to make buildings modular.