Gatsby believes he can erase time and reset "the clock" of his relationship with Daisy. He believes he and Daisy can pick up as if it were five years ago and she never married Tom and never had a child. As Gatsby says to Nick in a famous line, when Nick questions Gatsby's dream of recapturing the past:
"Can't repeat the past? ... Why of course you can!"
The theme of stopping time had been symbolized earlier when Gatsby knocked over the stopped mantelpiece clock at Nick's house and caught it in his hand. At that moment, his first encounter with Daisy in five years, time does seem, to him, to have stopped.
Nick relates Gatsby and his quest for Daisy to the American Dream. To Nick, Gatsby exemplifies the American Dream, the idea that we can remake and right the past. To Nick, this dream that drives us is a dream, not a reality, one that leads us on an impossible quest that has always "eluded us, and yet "we beat on, boats against the current..."