Calendar
A calendar is a system of organizing days for commercial, social, religious or administrative purposes. This is done by giving names to periods of time, usually days, weeks, months and years. Periods in a calendar are typically synchronized with the cycle of the sun or the moon.
What does AD stand for?

A.D. stands for the Latin phrase Anno Domini meaning Year of the
Lord and it dates from the arbitratary birth of Christ.
It was introduced by the monk Dennis the Short (Dionysius
Exiguus) in about 525. The date of the Incarnation was recognised
by the St Bede the Venerable to be inaccurate by about five years
however as it was in common use by this time the error was allowed
to stand.
Which century are we in?

We are in the 21st century and will be through the year
2099.
That depends.
Those for whom it is 2008 are in the 21st century
of the Common Era.
Most people say that this started at January 1st 2000.
Sticklers for correctness say that it started on January 1st
2001.
Jewish people are in the year 5769 - so they are in
their 58th century.
Muslims are in 1429 AH, their 15th century.
Other calendars are in different years.
It is the 21st century.
We are currently in the 21st century. It became the twenty first
century in the year 2001. When the year 2101 comes it will then be
the 22nd century. Every one hundred years marks a new century.
we are in the 21st century & you are here too.
the 21st century