Time Vault

Unlimited Edition
Alle series
Collectors’ Edition
Intl. Collectors’ Edition
Limited Edition Alpha
Limited Edition Beta
Masters Edition IV
Vintage Masters
Unlimited Edition
Time Vault enters the battlefield tapped. Time Vault doesn't untap during your untap step. If you would begin your turn while Time Vault is tapped, you may skip that turn instead. If you do, untap Time Vault. T: Take an extra turn after this one.
Banned in: Commander, Duel, Legacy
Restricted in: Oldschool, Vintage
2004-10-04: If multiple “extra turn” effects resolve in the same turn, take them in the reverse of the order that the effects resolved.

2008-10-01: The choice of whether or not to skip a turn is made as that turn would begin, and only if Time Vault is tapped at that time. If you choose to skip your turn, Time Vault untaps before anything else happens in the next turn.

2008-10-01: This wording means that you no longer end up skipping multiple turns if you find a way to activate the ability multiple times.