Once you're well-known and respected enough, you get to decide when historical eras start and end!

As an example, pop historian extraordinaire John Julius Norwich picks 1400 as the start date of the Venetian Renaissance, and offers a two-part justification: (1) that year, the new Doge's Palace was completed, and (2) it is a nice round number.

We can, of course, question the good Viscount Norwich's decision. Plans for the Doge's Palace were naturally drawn up in the decade prior to its completion, so is it really the first Venetian Renaissance building or is it rather the last Venetian Gothic building? How representative is the building of the political, cultural, and artistic principles of the renaissance, and what was its role in divulging them? (the argument that 1400 is a nice round number, on the other hand, is fairly airtight)

Was the renaissance even a Venetian phenomenon? Is it not better represented at some later date when Tuscan social and artistic principals disembarked in Venice? Can we jump from the High Middle Ages to the Early Modern period when allocating events in Venice within the macro-narrative of European history? Who decides this?

If I were to ask the question to one of my former professors, they would answer "I do not care, because it does not matter." We would get the same answer if we were to ask about the Early Modern Period instead of the Renaissance, or any other historical period. This is because even if a well established historian can argue that a given historical period truly emerged at a certain date, they seldom do. In fact, in the rare instances when generally-accepted boundaries of periodization do move about, it's often the result of decisions taken without explicit justification (although those justifications might be implied).

This occurs because historical writing does normally need to establish boundaries, for convenience's sake if not for anything else. And periodization offers a quick and easy way to establish boundaries. These boundaries can be implied even if precise start and end dates of historical periods aren't defined. In fact, because the exercise of defining them with precision can be tiresome and often adds very little value, "commonly accepted" dates are used even if they are more often than not tractable to customs established by nineteenth or twentieth century historians working with enormous biases and long outdated methodologies. When Gibbon decided the Roman Empire fell with the deposition of Romolus Augustulus, is that accurate? Who knows, but today it's both a convenient milestone to keep using and an equally easy milestone to debunk: you could argue, persuasively, that a sequence of trends and customs accepted as belonging to Late Antiquity "truly" ended at a later milestone, such as Justinian's reconquest. And an even more difficult date to agree on is the precise start of Late Antiquity. But the point is, do we really need to enter in this argument in order to perform historical analysis?

As long as the way we are using periodization is clear, insisting on precise start and end dates for historical periods isn't necessary. In fact, the very benefit of periodization is that it allows you to frame events without having to precisely define anything at all, even of those events are on the cusp of one commonly used period transitioning into another: We can speak of the Italian Wars of the late 15th Century, for example, as an "Early Modern Conflict" and this would communicate that we interpret the Italian Wars as a phenomenon that established and/or projected forward the values, dynamics, and trends accepted as belonging to the Early Modern Period. Most likely, this stance would frame the Italian wars as the first Early Modern milestone. But we could construct an equally justifiable stance in which frames the Italian Wars are the last renaissance milestone, and thus imply our view is projected backwards. But in both instances, the objective of the periodization is to clarify where a certain event (or interpretation of a certain event) fits into the historical narrative we are building.

This is a subjective question, beging that different people will give you different time periods.

In general, the Early Modern Period begin around 1600, though some historians might put it around 1500. It ended around 1800, one of my professors put it at around 1750 while another put it at the end of the Napoleonic Wars.

Then followed the Industrial Period from around 1800 to the World Wars. The World Wars are kinda treated as its own thing, but you can put a end date to the Industrial Period kinda anywhere though most put with WWI. Then comes the Modern Period which, in general (again) from about WWII to recent history.

No one person really decides where to begin and end a period of time. Most just look at general trends in society and how it changes. When a society changes enough then it becomes a different era. (Example: 1300 and 1600 society looks nothing alike. In 1400 there are small trends that are in 1600 but not enough to say there was a significant change from 1300. 1500 has many trends that 1600 has but retains some stuff from 1300 this making it a gray area when setting the date.)

And this is all from a European/Western perspective.