Also, computer systems have different Gregorian calendars. Your system could use the proper Gregorian calendar and a system you're communicating with could be using the proleptic Gregorian calendar. Most people don't notice the difference because dates aren't frequently slung around < 1582 in the modern world, but if a system has an idea of an "unset" date being equal to 1/1/1 it could lead to an error if transmitted to the other, resulting in an invalid date 12/30/0.
Not to mention dealing with the numerous differences in when the Gregorian calendar was adopted, which is great fun if you need to deal with dates even back to the early 1900's across borders.
E.g. the "October revolution" falls in November - Russia didn't change until the Bolcheviks took power in 1917. And Greece didn't switch until 1923... China switched in 1912, but different factions in the civil war used different systems and it wasn't until 1929 they got a single (Gregorian) calendar again.. And there are many other countries that switched "recently".
