What is "calendar"

There are several calendars in use in the world, the most widespread and currently used is the gregorian. But there are also others, such as coptic, islamic, hindu, among others. Each has different rules to define the day, month and year, from the initial value (which is the zero year of each calendar), to the duration of the months and years. Some calendars have 13 months, for example.

Most programming languages work with gregorian calendar for default, since it is the international civil calendar in fact, and is what most applications use around the world. But many languages also support other calendars, such as Java, the C# and the PHP, for example.