Time was a measurable part of the structure of the universe, in which the galaxy existed, where events occurred in a sequence. Time was measured with different concepts, such as millennia, centuries, and years. Time could be broken down even further into months, weeks, days, hours, minutes, and seconds, though these divisions were used almost exclusively to refer to the duration of events, rather than the date or time on which they took place.
Time was tracked on chronometers.
The last great conflict between the Jedi Order and the Sith Lords resulted in a definitive Republic victory. Resetting its capital on Coruscant, history reset its chrono on this date from which the modern Galactic Republic was born. The time before this rebirth was forgotten as a dark age, lumped into a collective whole known as the "Old Republic." As a result of this change, official calendars reset their zero year onto this date. The galaxy used this standardized dating system based off of the galactic capital Coruscant. On Coruscant, a year was made up of twelve months spanning 365 days of 24 hours each, with no leap years.
Sometime after the Battle of Yavin, Ithorian artist Gammit Chond used a calendar system that used the battle as the zero year from which to date galactic history. Dates before the Battle of Yavin were marked "BBY" and dates after the Battle of Yavin were marked "ABY."