A nitpick sixteen years too late: the idea behind five-digit years isn't bad, but there's no reason for time deltas to adhere to this rule! 5000 years ago, not 05000 years ago.
did the crossing of the rhine take place in 00406, 0406, or 406? what extra information do the two former styles convey?
also, what about the year 100000?
we are somehow doing just fine without leading zeroes for other quantities.
there is an argument to be made about e.g. iso8601 datetime formats that need to be lexicographically sortable; but i don't see any of the longnow fans using anything like those.