It's documented. >A Date object can be created with an optional argument, the day of calendar reform as a Julian day number, which should be 2298874 to 2426355

It's documented. >A Date object can be created with an optional argument, the day of calendar reform as a Julian day number, which should be 2298874 to 2426355 or negative/positive infinity. The default value is Date::ITALY (2299161=1582-10-15).