Seal of Time

Basic Stuff
The Seal of Time is a powerful lock which takes a temporal surge to remove. It is usually found on doors in which Chronius waits behind as a boss.

C.I. The Game
On the Ark of Chronius, the fifth door is sealed. The temporal surge used to unlock this seal is re-defeating all the bosses beaten prior to gaining access to the ark. This is a temporal surge due to already defeating them and needing to do the same thing twice. The minority of this type of temporal surge means this particular seal is weak, though still impenetrable without beating them again.

C.I. The Second Coming
In The Ruined Metropolis, there are 2 doors in the last area; the first has the seal, the second doesn't. In the second door, you must fight a MiniBoss which appears to be Chronius, but is actually a Temporal Ranger. After beating it, it releases a temporal surge, which, as mentioned before, unlocks the seal on the first door. Behind this door is the Boss (though not an entire boss, per se) Hand of Pyrax.

C.I. The Trickster's Trickery
Chronius doesn't appear in this game, however The Joker places these seals around various places, usually just before Boss Fights. The seals in this game, however, are unlocked by finding the Key of Chronius in the level in which the seal is in. There is one on the door in Playful Forest, hiding the Boss Jeleki, one on the door in Rough Rocks, hiding Kejii, one on the door in Rampaging Ruins, hiding Gadgadgi, one on the door in Hectic Peaks, hiding Moukkatu, one on the door in the Toy Factory, hiding Bakulakki, and one on the huge door leading from the Toy Factory (after beating Bakulakki) to The Joker which is unlocked by defeating minibosses resembling the bosses from previous levels, except Bakulakki. The last one, as stated, is unlocked similarly to the one in the first game.