\begin{definition}[Definition:Calendar/Gregorian]
The '''Gregorian calendar''' is the calendar which was introduced by [https://en.wikipedia.org/wiki/Pope_Gregory_XIII Pope Gregory XIII] in $1582 \, \text{CE}$.
Over the course of the next few centuries it was gradually adopted by the various subcultures of [https://en.wikipedia.org/wiki/Western_culture Western civilization].
It was derived as a refinement of the Julian calendar to correct for the discrepancy between the Julian calendar and the tropical year.
The years themselves are assigned the same numbers as their Julian counterparts.
An evolving modern convention is to refer to $\text{A.D.}$ and $\text{B.C.}$ as $\text{CE}$ and $\text{BCE}$, for '''common era''' and '''before common era''' respectively.
Like the Julian calendar, it divides the year into either $365$ days or $366$ days, according to the year number.
The months, similarly, are kept the same as for the Julian calendar.
The only difference is in the determination of which years are classified as leap years.
Let $y$ be the year number.
Then $y$ is a leap year {{iff}}
:$y$ is divisible by $400$
or:
:$y$ is divisible by $4$ and $y$ is not divisible by $100$.
Thus, for example:
:$2016$ was a leap year, because $2016$ is divisible by $4$ and not divisible by $100$.
:$1900$ was '''not''' a leap year, because $1900$ is divisible by $100$ but not $400$.
:$2000$ was a leap year, as $2000$ is divisible by $400$.
\end{definition}