I'd always been exposed to the definition of Early Modernism as roughly "between Westphalia and the French Revolution," or maybe starting with either the beginning of the English Civil Wars or the Restoration if talking specifically about Britain. ~1648-1789. This definition meant that the period focused on, among other things, the near-constant Great Power warfare in that era, the global expansion of European power, Baroque, Rococo, and Neoclassical art forms, absolutism, and the rest of the list.
I'm seeing more and more definitions of "Early Modern" that are hailing back as far as Martin Luther's 1517 defacing of a church door that made a few people upset. Bringing the entire 16th century and the Thirty Years War into the Early Modern period is deeply confusing to me because the periodization I've always been familiar with treated the end of the Thirty Years War as the start of that period. Are Calvin, Hermes Trismegistus, Paracelsus, the Schmalkaldic League, the Battle of Alcácer Quibir and union of Spain and Portugal, and Lepanto now "Early Modern" topics?
I know all periodization is arbitrary, but it has been throwing me for months now to see references to "Early Modernists" studying the 16th century. Am I just behind the times here?
I've never seen 'Early Modern' begin with the the Treaty of Westphalia. Where did you see/read that? What were the preceding 150 years seen as? Medieval?
But yes, although the periodisation varies from country to country and field to field, it's usually somewhere between 1450 and 1550. The most commonly used 'universal' markers are the three mentioned by /u/captainkaba (Fall of Constantinople, 'Discovery' of America and Luther's 95 theses). The latest date I know of for the end of the medieval period is from Ireland, where the end of the Geraldine rebellion led by 'Silken' Thomas, 10th Earl of Kildare, in 1537 is the standard date. The earliest is in Italian history, where I have seen the Middle ages cut off in 1348 (end of the first Black Death outbreak), with a period of 'Renaissance' followed by the Early Modern proper beginning in the 1490s with the French Wars.
The beginnings and endings of epochs vary widely not only from field to field (like ecclesiastical history, political history, ..) but from country to country as well, as you guessed. This is most obvious in art history, as different styles get big at different times (in early modern timeframe, italy for example was quite a bit faster than Germany or France).
Nowadays, in Germany it is commonly accepted to see the early modern period start with either
You cannot draw a simple line, but you could say that somewhere between these dates, a new epoch starts to rise. This transition is happening at many small levels, hence the unavailability of this one date. Mercantilism/kapitalism, printing, universities, and nationalization (at a very small scale in the beginning), and so on.
Terminologies such as the early modern period is defined by convention, and most agreement is that this is the period covering the Reformation, the age of European exploration, and the rise of the modern state system which had a lot to do with the shift into gunpowder weapons.
I have read a few instances where the Era is limited to post_Westphalia, but this is in the minority. There is some disagreement as to whether the sea should refer to the changes in what became the modern world, or the upheavals that dismantled the end of the medieval period, or both. I tend to agree it should cover both.
/u/DonaldFDraper says the period before Westphalia is too messy, I say the period following Westphalia is too linear. Har Har Har.
In the context of studying English literature and the development of the English language I took 'early modern' to refer to the period between the Battle of Bosworth which started the Tudor dynasty and the Act of Union which joined England and Scotland.
The boundaries are of course much more fuzzy than that - but that was a good enough rough guide for the stuff I was looking at.