I've heard several arguments that the post-modern era began during such periods as: WWI, the mid-20th century with the rise of mass consumerism, the 1980s, and the post-cold war era.
What defines the post-modern world other than it being "after the modern world"?
I'd like to tag on a more "meta" question to this discussion: can we define a "post-modern era" at all?
In my studies, "post-modern" always referred to theoretical movements within the arts (and even then, these movements were only part of what was going on). Can "post-modern" define an era?
Immediately after World War II is probably the flag for the beginning of the post-modern mood as it was not something which universally occurred across the humanities and sciences. It was also more popular in the continental traditions of thought, and largely criticized in the analytical.
One of the best descriptions of this period of time, I have heard, was that academia lost it's fury, especially in America. Modernism's utopian mindset was considered a failure, and the ideal values of that period were now under harsh scrutiny. We began to question absolute truth (relativism) and the meanings of theory and belief (anti-normativity). Culture was a well-believed myth that our ancestors created; society was a driven by materialistic desires and men profiting from rabid consumers; and history was just a narrative of wars and conflict driven by powerful men. Post-modernism challenged it all: deconstructing, critiquing, shredding it apart. Intellect was for cutting rather than creating, as Foucault put it.
This period of time has largely ended. There is still an echo of it remaining in some parts of higher education; but mostly seen in social movements ignited by those theorists (Feminism, Queer Theory, Critical Race Studies, etc.) Personally, I still see distrust for ideologies and moral arguments throughout our society. I also believe consumer-capitalism has largely displaced the humanities as irrelevant because of this age (among other things).
What good came out of the mood is largely up for debate, and usually dependent on who you ask. A lot of critique of social norms, such as sex and gender, did stem from the tradition; however contemporary studies are moving away from their postmodern since they lack substantial foundations. Power Theory in political and social science could be seen as windfall from this time. The term "social construction" can be added to that as well.
The bad, in my opinion, may be the hindrances of cognitive linguistics and capacities theory. Widespread acceptance of moral relativism and late existentialist nonsense. Fredric Jameson talked about this period's sheer refusal to critically engage in cultures, histories, and social reason, which led to poor multiculturalism and puerile respect for other socities. Much of what was said is just all-around obtuse, obscure—blurring the lines between our beliefs but never trying/desiring to piece together what they deconstructed.
When it comes to writing history, I think postmodernism and its methods are much more important than a neatly defined post-modernist 'era'. There was an excellent recent answer on this over in /r/AskLiteraryStudies, which made the important distinction of postmodernism and postmodernity. Not written from a historical perspective, but I found the explanation very insightful.
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