Wikipedia
Yavin is a fictional planet in the Star Wars universe. It has a satellite system and is best known for the Battle of Yavin over one of its moons, Yavin 4, depicted in Star Wars: Episode IV – A New Hope.
The Galactic Standard Calendar was the standard measurement of time in the Star Wars galaxy. It centered on the Coruscant tropical year. The Coruscant solar cycle was 368 days long; with a day consisting 360 NET degrees (or 24 standard hours). Numerous epochs were used to determine calendar eras. The most recent of these calendar eras used the Battle of Yavin (i.e. the destruction of the first Death Star) as its epoch, or "year zero": BBY (Before the Battle of Yavin), and ABY (After the Battle of Yavin). The earliest date in the Expanded Universe as a whole is 13,000,000,000 BBY, which serves as the year the universe was created.