An Era is a period of time that is associated with a particular quality, distinctive character, events, person, etc.

The time when new changes are born or activated into the natural earth and the influence that they cause over human life, beginning from that point of time as they progress through the stages of upgrade, development, civilization etc.

b : a memorable or important date or event; especially : one that begins a new period in the history of a person or thing.

An Era is also referred to as a calendar Era due to the fact that when a new Era begins this also affects the counting of time, date, months, years on a calendar.

For example;
The Christian Era based on the traditionally reckoned year of the conception or birth of Jesus (Yeshua) of Nazareth on Gregorian calendar. 

The Roman Gregorian calendar was designed according to the event when the star appeared up over Bethlehem exposing the new birth of a new born Child.

The Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table to reflect the Roman christian Era which reckons it’s beginning from the star which appeared over Bethlehem. 
His system was to replace the Diocletian era that had been used by Rome in an old Easter table.

That Roman Christian Era on the Gregorian calendar ended in the year 2012 in the month of December. From the year 2013 the Messianic Era began.

Article written by Ap Ngab Alex