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.