The Millennium Calendar is a mythic Legendary Artifact card from The Lost Caverns of Ixalan (LCI), collector number 257. Current market price: $3.47 (foil $5.32), TCGplayer market prices via Scryfall.
Whenever you untap one or more permanents during your untap step, put that many time counters on The Millennium Calendar. {2}, {T}: Double the number of time counters on The Millennium Calendar. When there are 1,000 or more time counters on The Millennium Calendar, sacrifice it and each opponent loses 1,000 life.