Cheapest priced copy (The Lost Caverns of Ixalan): $4.99 at Card Kingdom, near mint, their price list as of .
The Millennium Calendar 
Legendary Artifact · Mythic · The Lost Caverns of Ixalan Promos · #257p
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.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
4 printings · $4.99 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan | 257 | Mythic | $4.99 | $4.99 |
| The Lost Caverns of Ixalan | 388 | Mythic | $4.99 | $6.99 |
| The Lost Caverns of Ixalan Promos | 257p | Mythic | $9.49 | $7.99 |
| The Lost Caverns of Ixalan Promos | 257s | Mythic | — | $9.99 |
2 rulings from Wizards of the Coast
- 10 November 2023If 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.
- 10 November 2023The 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.