Certain concepts and creations by some of Lovecraft's friends are also used, and since original publication the circle of inclusion has gradually widened, in imitation of his own widening circle of correspondents. The Cthulhu Mythos transcends all time and space, but certain arcane books may be yet unwritten or unpublished in other centuries. Most Call of Cthulhu scenarios are set in the United States, in the 1920s or the present day. The latter half of the 1920s and the first half of the next decade were Lovecraft's most productive period. Others find the 1920s too strange, too plodding, and too lacking in firepower, and move games forward into the modern era, where bullet holes can aid the air conditioning.
Supplements exist for each era.
The eras are distinguished when the rulesbook refers to historical information, but one era is not better than another. Set play when you wish. Make a new era if you like. Historical settings are as real as possible. The world and the United States were very different in the 1890s and 1920s from now, and behaviors most find repugnant today were ordinary and acceptable then.