Why dates are important in history?

Dates are important, as they note when certain events happened. This is very important because history is recorded chronologically. It helps to know that one event happened before another event so that one can examine the relationship between events. Dates also serve to mark periods in history. While historians can disagree on exact starting and ending points for periods—for example, some historians do not put the exact starting date for the Middle Ages at 500 AD—these dates can be used as rough guidelines for looking at trends in history. The people who lived during those time periods did not say that they lived in a certain time period; no one living in Europe in 1922 would have called those the "interwar years," since no one knew that World War II was in the future. Historians use dates in order to signify eras.
Dates are also important for the cultural identity of a group of people. July 4 is a very important date in United States history, as it is considered to be the founding of the country. Other dates such as December 7 and September 11 are used in America to remember the sacrifices of those who died due to foreign attack.

We can think of history as a long chain of cause-and-effect. Even seemingly minor happenings can have lasting repercussions, leading to larger and more important events. Without knowing the dates of these events, we might never be able to trace the important ones back to their beginnings.
Dates serve as boundaries and markers for different periods in history. Knowing the dates of events allows us to place them in chronological order on a timeline—if we only knew what happened in the past, and not when, we would not be able to easily identify the progress of ideas or see the relationships between them.
The Gregorian calendar is the most prevalent in the world today. We mark years as BC (before Christ) or AD (anno domini, i.e., “after the death of Christ”). However, there are several other timekeeping systems, including Hebrew, Islamic, and Chinese—none of which having to do with the life of Christ. When keeping track of historical events, though, it doesn’t really matter which calendar you use, as long as you put them in the right order.
https://eclipse.gsfc.nasa.gov/SEhelp/calendars.html