Is The Millennium Calendar legal in Standard, Modern, and Commander?

The Millennium Calendar is legal in: Standard, Modern, Pioneer, Legacy, Vintage, Commander, Historic.

What type of card is The Millennium Calendar?

The Millennium Calendar is a Legendary Artifact. Its oracle text reads: "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."