Call me thick, but I think some general date ranges to cover the Earth period settings would help, and perhaps list these in chronological order?
EDIT: More random thoughts. Some of the settings seem redundant, since they become automatically included in certain system games. Hm, though in answering my own question I suppose you could use the setting but not the actual system, or a different system.
'Imperial' is a confusing setting title. To me it implies the period of European Imperialism, say 1830s to 1939. 'Classical' might more clearly cover this period? That includes pre-Imperial Rome too and of course Ancient Greece/Persia et al. With a stretch you could call this the setting that links between Pre-History and the Dark Ages.
I do think dates will help here, perhaps included in the tooltips/popups?
ANOTHER EDIT: More random thoughts...
'Age of Gunpowder' - to many that might imply a period between about 1500 through 1850. Can I suggest you call this setting 'Medieval to Renaissance'.
'Historic' - somewhat confusing. Can I suggest 'Dark Ages'
'Prehistoric' - a bit picky but a I think a better term is 'Pre-History'.