We don't need to put 0s in front of dates before 1000. Why do we need to do it for dates before 10000? Any date after that will have at least 5 digits, which di

We don't need to put 0s in front of dates before 1000. Why do we need to do it for dates before 10000? Any date after that will have at least 5 digits, which distinguishes it from dates before then, which only have four digits.