Anyway, the standard library allows each Date object to have its own date of calendar reform.
That's right, a Date object doesn't just track whether it is a Julian or Gregorian date, but what day should be used as the Date of Calendar Reform in reference to it, so that if you add or subtract some days or months to or from it, it knows when to automatically switch from being a Julian date to a Gregorian date, or vice versa.
But think about that. Is it really necessary? I think it's about as helpful as a sixth finger.