Dates are important to history because they help chronologically show cause and effect relationships between events. By knowing exactly when things occurred, historians can also more accurately compare different societies at specific points in time.
Dates have been recorded by different cultures and groups of people starting at different times. Christian Europeans adopted the convention of recording dates by whether they occurred before or after the year Jesus Christ was born. This is the system most of the world uses today. When looking at dates earlier than year one, historians count backward and label the year BCE, or "before the common era."