Take the UK for another example. The daylight savings dates are actually set by act of parliament. Although they always have fallen in the pattern that everyone knows, for dates in future years beyond the ones they have already set, they could hypothetically (if you want to be literal-minded about it) change the law to make DST happen on absolutely any day of the year or not at all.

Sure, we cant know what unix date it resolves to, but it doesn't matter, because future dates are more of a contract intentionally bound to context that is subject to change.