But practically speaking, the consciousness of the times we live in determine which calendar has the most meaning. Therefore the Gregorian calendar is THE calendar in these times, and the turning of the millennium was a huge event--not the end--but the beginning of a quickening.
It's all consciousness. If consciousness agree that the sky is red, then it's red. If the predominate awareness says that the millennium is important, it becomes important--a self-fulfilling prophecy. That's how things work on a metaphysical level.