The definition of a
Dateis very simple: It’s a combination of the year, month and day fields, like (year=2012, month=12, day=31). However, the values of the year, month and day fields have constraints, so that the date value is a valid day in the real world. For example, the value of month must be from 1 to 12, the value of day must be from 1 to 28/29/30/31 (depending on the year and month), and so on.
These constraints are defined by one of many possible calendars. Some of them are only used in specific regions, like the Lunar calendar. Some of them are only used in history, like the Julian calendar. At this point, the Gregorian calendar is the de facto international standard and is used almost everywhere in the world for civil purposes. It was introduced in 1582 and is extended to support dates before 1582 as well. This extended calendar is called the Proleptic Gregorian calendar.
Starting from version 3.0, Spark uses the Proleptic Gregorian calendar, which is already being used by other data systems like pandas, R and Apache Arrow. Before Spark 3.0, it used a combination of the Julian and Gregorian calendar: For dates before 1582, the Julian calendar was used, for dates after 1582 the Gregorian calendar was used. This is inherited from the legacy
java.sql.DateAPI, which was superseded in Java 8 by
java.time.LocalDate, which uses the Proleptic Gregorian calendar as well.
Even in this three-paragraph snippet, you can already get a feeling for how complex working with dates can be. Then throw in the complexities of time and you get a detailed post full of good information.