From the Super Mario Wiki
+ Clocks (or Plus Clocks) are items that appear in many Mario platformers, where collecting them will give the player more time to complete a task. They first appeared in Super Mario Bros. Special under the name "Clock," where they add 100 units to the timer.
They reappear in Super Mario Galaxy 2, where their appearance is 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 only appear 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. Taking shortcuts in a mission will often cause players to miss them.
+ Clocks reappear in Super Mario 3D Land with a similar purpose and will refill the time left to complete the level. They appear in two different colors: blue and green. Blue clocks will refill 10 seconds of the time limit and green clocks will refill 100 seconds.
Both green and blue + Clocks also appear in the Nintendo 3DS game New Super Mario Bros. 2. Unlike the previous games, the green clock adds 50 seconds to the time limit when collected. The blue clock still adds 10 seconds. The + Clocks only appear in Coin Rush.