Agreed. However, it might be not so easy for historical dates, because doing it correctly requires great diligence on the part of the tool developer as well as from the user to choose the correct calendar system. For example:
Q: What is the correct representation of the date of Caesar's death, 15 March 44 BC in ISO-8601?
A: -0043-03-13
Why? -- Ancient dates are typically given according to the Julian calendar excluding a year 0, but ISO-8601 uses a proleptic Gregorian calendar including a year 0.