As near as anyone can pin it down, the Sumerians invented the predecessor of all written language (by which I mean symbols that were not pictograms, though they were often derived from such) approximately four thousand years before the beginning of the "common era". So, by simply adding four to the first digit of the CE year, we can enter a calendar which, in a fashion at least as accurate as the commemoration of the birth of Christ, commemorates the birth of writing.

I like writing, so I do this.

Ninety-nine times out of a hundred, one only needs to use the final two digits of the year anyway, unless one means to make historical reference or issue a cheque -- but, as history and cheques have ceded to prozac and plastic, the practice generally suffices.