This placeholder lists different epochs in human history and more vague but important timelines such as the future, the present and the past.
Anime that fall under this category are the ones that have a society that could have originated from ours. In most cases it should go hand to hand with SciFi since we expect the future to consist of advanced technology, however, in the case where it's explicitly stated in the story that the anime takes place "x years into the future", the state of technology doesn't matter as long as it's long into the future. The anime can be set in an alternative universe not tied with our own, as long as it has a futuristic feel to it.
An anime that takes place on future Earth (or it could be an alternative universe) after a major cataclysm has occurred. World War III, an alien invasion, plague, global warming or some other general disaster are common themes to build a post-apocalyptic world upon, where the effects were severe enough that mankind barely survived. The time frame may be immediately after the catastrophe, focusing on the travails or psychology of survivors, or considerably later, often including the theme that the existence of pre-catastrophe civilization has been forgotten (or mythologized). Post-apocalyptic stories often take place in an agrarian, non-technological future world, or a world where only scattered elements of technology remain. There is a considerable degree of blurring between post-apocalyptic fiction and that which deals with false utopias or dystopic societies.
Anime that takes place in the past or clearly seems to take place in the past in case of an alternative universe. This category should almost always go together with one of its sub-categories, because the past can either be a depiction of a historical event(s) or not, in which case it would be alternative past.
An anime that takes place in an alternative past is an anime that clearly does not belong to the future or the present, and isn't tied particularly closely to our history, or has ties to our history but only as a premise: there are too many things that don't make sense for the time-period. In some cases there is futuristic technology present, but the overall setting still feels that of the past.
Historical anime have clear ties to our history and usually occur in a particular epoch.
Bakumatsu - Meiji Era
The Bakumatsu period was between 1853 and 1867 during which Japan ended its isolationist foreign policy and modernized from a feudal shogunate to the Meiji government. It is at end of the Edo period and preceded the Meiji era. During this period, the main conflict was between pro-imperialist Ishin Shishi and the shogunate forces, including the elite shinsengumi. The Meiji Era (including Meiji Restoration) denotes the 45-year reign of Emperor Meiji, running from 23 October 1868 to 30 July 1912. During this time, Japan started its modernization and rose to world power status. The thing to note here, is that this category should be added only to anime that take place in Japan during the years mentioned and not to every anime that has samurai in it.
This category is for anime that take place during or around World War II, including the prelude to the actual war when Hitler came to power. Since Japan was actually involved in this war, there are more that take place in this period than in World War I.
The most common factor for an anime to take place in the present is that there should be recognizable things/events from our own society that both we and the characters take for granted. And obviously (unless it involves time travel or spans a long period in history) it cannot fall in any of the other time-categories.
Anime that fit in this category usually have technology and society that is very similar to ours, but with differences in both, history and culture, which sometimes are blatantly obvious, but sometimes harder to place; this one can at times be very subjective.