I swear jtur88’s post wasn’t on screen when I typed my post. Yeah, this.
Culturally, 1915 or so is probably a pretty decent pick. You really need the class system to have given way to the wealth system, I think, to call it modern. If you still have people fighting duels and marrying for titles, then it’s not modern. I feel like the classist systems lasted at least a bit into the 1900s, but by the flapper era were basically gone (even in Western Europe).
Modernism in literature does start with WWI. They’re probably onto something.
Since I have no concept whatsoever of “official” era definitions, and you foolishly asked my opinion anyway, I’d say that no era that predates a fully-realized internet could be considered “modern”. So I’d say, oh, around 1995 or so.
The previous era, spanning twenty years, was “The Eighties”. One notable attribute of the modern era is that virtually all pop culture is based on The Eighties. 
I think many people’s idea of the beginning of modernity is to implicitly start from their own birth date and to search backwards in the history books (or, less often, forwards) for a sufficiently important event to latch on to.
AND - I think that method isn’t so bad. Modernity is only about dividing “definitely the past” from “now-ish times” - isn’t that the case? I think our impression of “what are now-ish times” varies a lot, depending on who (and where) we are.
In what country?
But as a general answer I would say “modernity” is what you get when people believe that new things and new ways of looking at things, are likely to be superior to old traditional ways. So America and other colonial countries have been modern since colonisation. Europe - from around the industrial revolution, on a per-country basis. China - around the beginning of the 20th century. And so on.
Modernity is an attitude, not a technology.
I agree, but I would add “… an attitude to what?”.
For some people, modernity is primarily about things, and people’s relationship to the world. (Examples: industrial revolution, space flight.) For others, it’s primarily about people, and the ways people treat each other. (Examples: rise of democracy or end of legal organized slavery.) The two do often overlap.
Also, some things are regionally very important even if not so important elsewhere. It would be foolish, for example, to deny the social impact of air-conditioning in Arizona.
Though I’d agree that social progress is usually the sort of thing that people appreciate the most about modernity, I rather think that it’s a byproduct of the core concept that you can work, change things (technology, society, human psychology) and make things better. And that idea comes first from tech, even though I don’t think that things are the main feature that makes modern life worth living.
Random examples: Ancient Greece had (somewhat flawed) democracies, but I don’t think that makes them modern. Though they were probably closer to it than anyone else at that period - but it didn’t last, it didn’t form part of an actual sequence of progress in the way technological discoveries generally do.
Native American societies were quite egalitarian - that doesn’t necessarily mean they were ‘modern’ (in their thinking) either.
Saudi Arabia is technologically perfectly modern but socially, really not.
On the other hand, if you have a society where everyone is used to the idea of ‘progress’ and a lot of different people are considering how their society could be made better, ‘make it fairer and more egalatarian’ is obviously going to occur to a lot of them - because in a very hierarchical top-down society, there are more people whose lives will be improved by increasing egalitarianism than will be diminished. So there’s a connection there, just not an infallible one.
In the west? 1648 - Peace of Westphalia. Not really a traditional break, I think. But I like it as a personal preference. Never was that comfortable lumping the Renaissance into the early modern period and as a semi-arbitrary marker for the rise of nationalism I think it is a good harbinger of the [del]end[/del] modern times.
The first time a preponderance of the flock decided their own course outside of the dictates of their faith.
Therein lies the beginning of modernity for me.
Speaking of WWI, why did you mention 1915 instead of, y’know, 1914?
For me, the modern era follows the Renaissance, so it starts around 1650 (scientific discoveries, modern nation states and so on).
As a multiple of 5, it’s more clear that I was offering a general timeframe rather than a specific year. (I did say “or so”, after all.)
Not the reason that I said “1915” but I would also note that the Modernist writers who served in WWI mostly wrote after they got back from the war, not while on the battlefield. So the year following the end of the war would make more sense even if we were trying to peg it to the end of WWI.
Once the scientific method was formally worked out as a way of knowing the world there was no going back. So my arbitrary line of modernity occurs somewhere between Francis Bacon and Isaac Newton.
Another vote for World War One. It marked the end of absolute monarchies in West, sowed the seeds for the end of Western imperialism, changed the social order*, and saw technologies like aviation, internal combustion, and wireless communication really come into their own.
*For example, before and during the war British officers were expected to be wealthy enough to buy their own uniforms.