In the 75th Hunger Games ("The Quarter Quell") the arena is in the shape of a circle, with the cornucopia in the center, in the middle of a lake. Participants quickly discover the arena actually mimics a clock. This "clock" is important both concretely and symbolically.
The use of the clock ends up being very important to the characters in the games because for them, it is a method of survival. The way it works is two-fold. First, a new "torture" method is introduced by the game controllers on the top of each hour. These obstacles are also released in the geographical area pertaining to their time on the clock (for example, lightning is released at 12, in the area between 12 and 1 o'clock on the circle). The cycle is repeated in exact time, meaning each torturous event happens twice a day. Once the key characters discover how the arena works, they are able to use this knowledge in order to survive, and ultimately, to defeat others.
On a more symbolic level, the clock is also important. Remember that this Hunger Games is done "as a reminder to the rebels that even the strongest among them cannot overcome the power of the Capitol" (chapter 12). This clock, therefore, symbolizes the ideas that time is running out or that you cannot outrun time.
That said, this is the first Hunger Games that does not have a winner. It is in an ironic ending that Katniss (and a few others) are taken from the arena before the game ends. In this way, the clock never "runs out." Symbolically, the game does not end, just like time does not end. The audience is left, once again, on a cliff hanger, wondering if the tributes are being taken to a place of safety or something worse.