From the Super Mario Wiki
The + Clock (or Plus Clock) is an item that appears in some Mario platformers, where collecting it gives the player more time to complete a task. It first appears in Super Mario Bros. Special as "Clock", where it adds 100 units to the timer.
+ Clocks reappear in Super Mario Galaxy 2, as a green clock with a knob on top, with the minute hand pointed at it. These appear in several Speedy Comet missions and add ten seconds to the player's time limit, which starts at either twenty or thirty seconds. The player must collect these in order to complete the level. They appear only in the Speedy Comet missions for Sky Station Galaxy, Bowser's Lava Lair, Bowser's Gravity Gauntlet, Bowser's Galaxy Generator, and Stone Cyclone Galaxy.
+ Clocks reappear in Super Mario 3D Land with a similar purpose. They appear in two different colors: blue and green. Blue clocks refill 10 seconds of the time limit, and green clocks refill 100 seconds.
Both green and blue + Clocks also appear in the Nintendo 3DS game New Super Mario Bros. 2. Unlike in the previous games, the green clocks add 50 seconds to the time limit when collected. The blue clocks add 10 seconds. The + Clocks appear only in Coin Rush mode.
Green and blue + Clocks both reappear in Super Mario 3D World, where they add the same number of seconds to the timer as in Super Mario 3D Land. They once again appear to be somewhat rare in the gameplay. Like in Super Mario 3D Land, the blue clocks add ten seconds to the timer, while the green ones add 100.
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