\begin{definition}[Definition:Calendar/Gregorian/Year]
A '''Gregorian year''', also known as the '''civil year''', is the length of a year as defined using the '''Gregorian calendar'''.
:$1$ '''Gregorian year''' $= \begin{cases} 366 \, \text {days} & : 400 \divides y \\
365 \, \text {days} & : 400 \nmid  y \text { and } 100 \divides y \\
366 \, \text {days} & : 100 \nmid  y \text { and } 4 \divides y \\
365 \, \text {days} & : 4 \nmid y \end{cases}$
where:
:$y$ denotes the number of the year
:$4 \divides y$ denotes that $y$ is divisible by $4$
:$4 \nmid y$ denotes that $y$ is not divisible by $4$.
\end{definition}