I understand that people living in the Middle Ages cared more about surviving than taking selfies and posting them to Reddit, thus documenting every waking moment of every day. But surely there must be something that can be used to definitively state when a movement started.
The reason I'm asking this is because of the Middle Ages. I was curious as to when they started exactly. I Googled the question and Google immediately told me the Middle ages happened 500-1500 AD. Yet the first two links Google provided disagreed and claimed the Middle Ages started earlier than that and ended earlier as well. Why is it so hard to determine when the Middle Ages (and history in general)?
(To be clear, I'm not criticizing anyone, I just want to know what makes making timelines so difficult and imprecise.)
I imagine someone who works on the Middle Ages can speak directly to this question, but I'll just pipe in with some comments about periodization. Periodization is the process by which historians identify certain time periods as good candidates to be studied, (at least somewhat) independently from the surrounding years.
Some historical periods are very easy to identify. The interwar period is the years between World War I and World War II, and obviously there's no need for debate about when the interwar period occurred. That era has received more attention from historians over the past few decades because historians in various subfields have found it a useful tool to talk about events and issues important to them -- religious history, cultural history, or whatever. The interwar period has currency among historians today because it offers a handy cutoff to talk about a variety of other things, in other words.
That criteria can be applied to any historical period. Does limiting the chronology in this way help me tell the story that I need to tell? Do these years cohere in some way that also help to support a coherent historical narrative? But not all periods can be as sharply defined as the interwar years, which are bookended by two major historical events. Historians disagree about when the Middle Ages began and ended because they disagree about what the Middle Ages is. However, I'll have to leave the nature of those disagreements to someone with more knowledge about those debates.