The short novel The Great Gatsby was set in the context of changing times and starkly contrasted the era of the austere Great Depression against the glittering glitzy times of luxury enjoyed by the super rich. Nick is philosophical about the past; he tells Gatsby that it is not possible to repeat past times and the two characters seem preoccupied by this theme. Gatsby on the other hand is more expansive and optimistic, declaring "Of course you can!" Nick is more realistic and has a better grasp of the way things are going both globally and more locally. Gatsby is too involved with Daisy and the way life used to be in the old times, a time of carefree extravagance, indulgent ease and beauty, to care. Of course, his vision of this luxurious decadent lifestyle is colored by Daisy's place in it and is rose-tinted and misty, a little like the beautiful soft billowing of the gauzy curtains at the huge windows above the green lawns of the opulent house.
Meanwhile, Nick is obsessing with how it is all going to end, so in this we see that his issues concern both the past and the future and the threats and horrors the latter could bring. He knows that Gatsby's assertion that those with money can conjure up whatever times they like is flawed and that Gatsby spends his time trying to relive a shiny happy past with Daisy. Gatsby seems to believe that it is quality of time and not quantity that matters, that a glorious few weeks of vacation with the person you love is worth a lifetime of lonely boring misery. Gatsby doesn't seem to consider the possibility that money, as well as time, can run out.
Nick, however, seems to sense that it's all going to end and this fragmentation also shows in his relationship with Gatsby. They quarrel at the hotel and Nick has a sudden realization of what the time is in his own life. He is thirty! He perceives the next part of his life as a threatening and worrying time and can see how both new wealth and old inherited wealth will be affected by the coming economic crash.
The end of the novel is like an elegy and illustrates the author's own views. He sorrowfully contemplates the loss of what used to be and how things might have been if America had stayed as green and fresh and vibrant as the lush lawns of the big house. Instead "The American Dream" was soiled by greed like a flag trampled into the mud of 1920s American society.