Confidence:StrongSolidPlausibleTentativeSpeculative

• Paradox Zone

  4G

  EnchantmentG

  Counter synergy: both cards work with the same kind of counters, so their effects stack in a counters deck. Not necessarily a closed loop on its own.
• Intellectual Offering

  4U

  InstantU

  Intellectual Offering untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Unstoppable Plan

  2U

  EnchantmentU

  Unstoppable Plan untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Seedborn Muse

  3GG

  Creature — SpiritG

  Seedborn Muse untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Unwinding Clock

  4

  ArtifactC

  Unwinding Clock untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Paradox Engine

  5

  Legendary ArtifactC

  Paradox Engine untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Tekuthal, Inquiry Dominus

  2UU

  Legendary Creature — Phyrexian HorrorU

  Counter synergy: both cards work with the same kind of counters, so their effects stack in a counters deck. Not necessarily a closed loop on its own.
• Dramatic Reversal

  1U

  InstantU

  Dramatic Reversal untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• Blinkmoth Infusion

  12UU

  InstantU

  Blinkmoth Infusion untaps all (or many) permanents, refreshing The Millennium Calendar's tap activation.
• The Great Goblin

  1B/RB/R

  Legendary Creature — Goblin NobleBR

  Both cards trigger when a counter is placed — they fire together.
• Kate Stewart

  1URW

  Legendary Creature — Human ScientistRUW

  Both cards trigger when a counter is placed — they fire together.
• All Will Be One

  3RR

  EnchantmentR

  Both cards trigger when a counter is placed — they fire together.
• Rings of Brighthearth

  3

  ArtifactC

  The Millennium Calendar has activated abilities that aren't mana abilities, fueling Rings of Brighthearth's activation-trigger.
• Kurkesh, Onakke Ancient

  2RR

  Legendary Creature — Ogre SpiritR

  The Millennium Calendar has activated abilities that aren't mana abilities, fueling Kurkesh, Onakke Ancient's activation-trigger.