[Xietor]

All you have to do for the correct answer is read the unit's description of undead. Undead units are not affected by the burden of time because-drum roll-they are already dead.

That is what it means to die, and then have some foul necromancer raise up your bones, and imbue your bones with enough magic to make them move.

But since your flesh is gone, you cannot be poisoned. Since you are already dead-you cannot be diseased.

And because you are already dead, you do not care what day of the week it is, or even what year it is. You do not care what time it is-because the passing of time no longer affects your magically animated bones.