So you say, you can predict? That you have software which puts out very accurate predictions. Doesn’t matter if it is closed or open source. Freeware or proprietary. Made by NASA or by yourself.
Give it a file with all the dates between January the first 1600, and the last day of December 1999. That makes 146097 dates.
If the prediction program can’t tell you, that the next will be the first day of the year 2000 – a still unobserved value – then this software of yours is no good.
Of course, if the Gregorian calendar is hard-coded in to this oracle, then it’s merely a fake.
Just some advice for calibrating things.