When Time Elemental attacks or blocks, at end of combat, sacrifice it and it deals 5 damage to you.
{2}{U}{U}, {T}: Return target permanent that isn't enchanted to its owner's hand.
If Time Elemental attacks or blocks, but you no longer control it at end of combat, you can’t sacrifice it. However, it will still deal 5 damage to you.
A “permanent that isn’t enchanted” is a permanent with no Auras on it.
You both sacrifice it and deal the 5 damage at end of combat.