The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 8 when the quadrennial leap year stabilized.
The leap years that were actually observed between the implementation of the Julian calendar in 45 BC and AD 8 were erratic (see the Julian calendar article for details).
The Julian calendar itself was introduced by Julius Caesar, and as such is older than the introduction of the Anno Domini era (or the "Common Era"), counting years since the birth of Christ as calculated by Dionysus Exiguus in the 6th century, and widely used in medieval European annals since about the 8th century, notably by Bede.
The proleptic Julian calendar uses Anno Domini throughout, including for dates of Late Antiquity when the Julian calendar was in use but Anno Domini was not, and for times predating the introduction of the Julian calendar.
Before then, the Roman Republican calendar used a system of intercalary months rather than leap days.