In The Great Gatsby, time is a leitmotif that runs throughout the novel. It is mainly associated with Gatsby and his quest to repeat the past and reestablish his love affair with Daisy.
Observe this passage on page 110 regarding Daisy:
"I wouldn’t ask too much of her," I ventured. "You can’t repeat the past."
"Can’t repeat the past?" he cried incredulously. "Why of course you can!"
He looked around him wildly, as if the past were lurking here in the shadow of his house, just out of reach of his hand.
"I’m going to fix everything just the way it was before," he said, nodding determinedly. "She’ll see."
Gatsby's goal is the turn back time, to go back to his boyish days in Louisville when he first met Daisy, before the war, before she was married, before he became corrupted, back when America was the idealistic land of opportunity.
Daisy knows that the past cannot repeat itself. Nick knows this too. But, when Gatsby and Daisy meet at Nick's for the date, Gatsby thinks time is moving backwards. Observe the symbolism of the clock:
Gatsby, his hands still in his pockets, was reclining against the mantelpiece in a strained counterfeit of perfect ease, even of boredom. His head leaned back so far that it rested against the face of a defunct mantelpiece clock, and from this position his distraught eyes stared down at Daisy, who was sitting, frightened but graceful, on the edge of a stiff chair.
“We've met before,” muttered Gatsby. His eyes glanced momentarily at me, and his lips parted with an abortive attempt at a laugh. Luckily the clock took this moment to tilt dangerously at the pressure of his head, whereupon he turned and caught it with trembling fingers, and set it back in place. Then he sat down, rigidly, his elbow on the arm of the sofa and his chin in his hand.
“I'm sorry about the clock,” he said.
My own face had now assumed a deep tropical burn. I couldn't muster up a single commonplace out of the thousand in my head.
“It's an old clock,” I told them idiotically.
I think we all believed for a moment that it had smashed in pieces on the floor. (86-87)
Overall, the leitmotif of time shows the false idealism of Gatsby and the American dream. After World War I, Fitzgerald says that the innocent, young, boyish America of our past and of our dreams is changed, corrupted, and no more.