Other eras (the past)
While there are alternate futures, I think there's only one "true" past. I'll try to divide them into sections
Distant Past (before 100, 000 BC)
Pre-Cataclysmic Era (Kull Characters, pre sinking of Atlantis, @ 100, 000 -18, 000 B. C., and prior)
Hyborian Era (Conan characters, @ 18, 000 - 8, 000 B.C)
Post-Hyborian era through calender Birth of Christ (@ 7999BC - "0" BC/AD)
1st through 10th Centuries (1 AD - 1000)
Age of Camelot (Arthurian characters, 6th Century)
11th through 17th Centuries (1001-1699)
18th and 19th Century, non-Western
Old West (Cowboy type characters, America, 18th and 19th Century)
World War I (covering from 1900-1929)
World War II (covering from 1930-1945)
Post-World War II (covering 1945-1959)
Pre-Modern (covering 1960-just before Fantastic Four I#1)