From the Super Mario Wiki
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A Time Limit (or simply Time) is a staple of the main Mario series platformer games, and its basic purpose is to notify the player of how much time is allotted to complete a level or task; if the timer reaches zero, the player loses a life (and if the time reaches zero when the player has no more lives, he will recieve a Game Over). An in-game "second" is approximately 0.6 or 0.7 seconds (in Super Mario Bros., one in-game "second" lasts 0.4 seconds). When the time limit decreases to below 100 units of the time, the "hurry up" jingle plays to warn that the player is low on time and the level's music plays at a rushed tempo.
Once the player finishes a level, the remaining time is multiplied by a number and then added to the player's score (or coin total in Super Mario 3D Land). With few exceptions (e.g. in Super Mario World, Super Mario 3D Land, and Super Mario 3D World), it is impossible to increase a level's time limit. In Super Mario 3D Land, New Super Mario Bros. 2, and Super Mario 3D World, the time limit glows red when it goes below 100.
Time limits additionally appear in several other games such as in the Mario Kart series, Mario Party series, and various Mario sports games where they function as a simple timer for a given game session.