From Bulbapedia, the community-driven Pokémon encyclopedia.
The Time Gears (Japanese: ときのはぐるま Time Gears) are artifacts of great importance in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness and Pokémon Mystery Dungeon: Explorers of Sky.
The Time Gears help keep control of the flow of time in the Pokémon world. Even the most hardened criminals know not to disturb the Time Gears, lest the proper flow of time be disrupted in that general area. However, in the beginning of the events of Explorers of Darkness, Time, and Sky, a mysterious thief starts to steal them over the course of the storyline, causing time in those areas to stand still. Three of the five Time Gears are guarded by the lake guardians, and one of the other two is guarded by a Ditto. The other Time Gear is unprotected.
The thief is found to be Grovyle, who is from the future. Over the course of the games, the player eventually brings him to justice. The stolen Time Gears are then returned to their respective areas. However, it is then revealed that Grovyle stole the Time Gears to prevent an even greater catastrophe: the paralysis of the planet. It is also revealed that Dusknoir, a famous explorer believed to be a hero, is actually a servant of Primal Dialga, the main villain of the first storyline. Grovyle comes back from the future with the player to stop the paralysis of the planet. Grovyle explains that the frozen time caused by the missing Time Gears would be only in effect temporarily until they were placed in Temporal Tower, Dialga's domain, where the flow of time would be fixed permanently. The player then decides to help Grovyle collect the Time Gears again, and goes to the guild to explain the situation to the guild members.
When the player and Grovyle go to Treeshroud Forest to take the first Time Gear again, time has already frozen. This prompts Grovyle to act quickly to obtain the rest of the Time Gears. The party then goes to the Hidden Land, and from there, Temporal Tower. Grovyle throws himself and Dusknoir, their pursuer, into a portal going to the future, while he entrusts the five Time Gears to the player in order to prevent history from repeating itself. Together, the player and their partner place the Time Gears in the slots at the top of the tower after defeating Primal Dialga, effectively preventing a future of darkness. The story of Grovyle and Dusknoir is then continued in the Special Episode In the Future of Darkness. It is also later revealed that Darkrai was actually the one behind all of these events.
In the anime
A Time Gear being stolen by Grovyle appeared in Pokémon Mystery Dungeon: Explorers of Time & Darkness.
In Pokémon Mystery Dungeon: Explorers of Sky - Beyond Time & Darkness, Grovyle gave three Time Gears to Team Poképals when he was about to go back to the future.