The Millennium Calendar

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.

The Lost Caverns of Ixalan

The Lost Caverns of Ixalan Promos

If The Millenium Calendar's last ability is countered or otherwise removed from the stack, it will just trigger again immediately if The Millenium Calendar still has 1,000 or more time counters on it.

The Millenium Calendar's first ability triggers during the untap step. However, since no player gets priority during the untap step, the ability waits to be put on the stack until the upkeep starts. At that time, any "beginning of upkeep" triggers will also trigger. Those abilities and The Millenium Calendar's ability are put onto the stack at that time.