Why is Gatsby obsessed with time? How does he use time to construct opportunities for himself?

Jay Gatsby’s preoccupation with time is a prominent theme in the novel. Sometimes painfully aware of the distance he has placed between his own poor childhood and youth and the wealth he enjoys when Nick meets him, Gatsby wants to believe that his new status is enough to bring Daisy back to him, and permanently. Part of him knows that this is a dream, but he clings to it nonetheless. Finding that Nick is Daisy's cousin presents an opportunity, and he sets about wooing her with Nick’s assistance.
There are several significant places where Gatsby’s association with time is stressed. Nick’s accusation of trying to bring back what is already gone and Gatsby’s reply are primary among these.

“You can’t repeat the past.”“Can’t repeat the past?” he cried incredulously. “Why of course you can!”

Gatsby also understands, however, that money acts as a shield. One of the things it protects people from is aging, which he associates with poverty.

Gatsby was overwhelmingly aware of the youth and mystery that wealth imprisons and preserves, of the freshness of many clothes, and of Daisy, gleaming like silver, safe and proud above the hot struggles of the poor.

More generally, in discussing the promise of America, embodied in the Long Island landscape, Nick conveys that Gatsby’s vision is that of the old-time explorers, a profound but incomprehensible sense of enchantment and wonder.

For a transitory enchanted moment man must have held his breath in the presence of this continent, compelled into an aesthetic contemplation he neither understood nor desired, face to face for the last time in history with something commensurate to his capacity for wonder.