Historians use both the timeline and the calendar, but they are
used for different purposes. Calendar is used by historians for marking
specific important dates, like the date of birth or the date of death. Timeline
is mostly used to describe an incident that has occurred over a period of time.
This occurrence could have started in one era and ended in another. These kind
of incidents that occur over a longer time are mostly depicted by timeline. For
example the reign of a ruler can be depicted by a timeline by any historian.
