Why? It's still three numeric digits. If anything, it would be much easier to calculate date validity. In Python:
acceptable_days = range(1,29)
if pickedMonth == 14:
acceptable_days = [1];
if isLeapYear(year): acceptable_days = [1,2]
# any future hack will just go here
if pickedDay not in acceptable_days or pickedMonth > 14:
raise InvalidDateException()
Compare this with anything we use today. Which code is simpler?
I don't see how it would make things much easier, especially for timestamps in various date formats and date-input fields.