Hillel the Second established the Jewish Calendar in the middle of the fourth century CE because, due to the conquest of the Land of Israel by the Romans, it was no longer feasible to rely on the Sanhedrin to know when the festivals were. Hillel the Second made the calendar for two thousand years and it will require a Sanhedrin to renew it but there is still time until the calendar expires.
The concept of six thousand years is very important in Judaism because there is a verse in the Book of Psalms that states that a thousand years is like a day in the Eyes of God. There is a Kabbalisitc concept that the world will exist in its current state for six thousand years, which mirror the six days of creation, and then the Messianic Era will begin which will resemble Shabbat.
