An universal calendar
Le calendar almost universally used is the Gregorian calendar and its making for the next years needs calculations coming from astronomical considerations. This calendar and its movable feasts are provided in France by LTE under the auspices of Bureau des longitudes and Observatoire de Paris. More explanations are provided here. The Gregorian calendar is a solar calendar linked to seasons (one year in the Gregorian calendar approximates (nicely) the « tropical year » and one month approximates (badly) a lunation). The dates of the « movable feasts » depend on the motion of the Moon for religious reasons.
The [calendar for the current and next years is available here.
Identifying dates
The identification of moments requires the definition of time scales, but regardless of the choice of a scale, it is necessary to define a system of numbering days. It is easy to see a change of day, it is more difficult to find a change of year and the day is the natural unit. Astronomers number the days since Monday, January 1 -4712 at 12 hours, it is the "Julian period." This numbering system is designed to locate any time without ambiguity: it solves the problem of chronology.
For reasons of tradition, but also convenience, one was taken to use a more complicated system by cutting in years and months. Adopted rules that constitute the various calendars differ from one civilization to another.
Matching several calendars
Several calendars are used in the world and one will find here the correspondance between calendars.