The speaker of Sonnet 18 is absolutely fixated on fate and mortality, but believes he’s come up with an effective time machine: poetry. Instead of contemplating a beautiful summer’s day, this speaker can’t stop thinking about how everything in life is temporary and fleeting. No need to fear, though – the hero-poet steps in and announces that, by artistically representing his beloved, he can save him or her from the ravages of time. "Time," then, is the intersection of the "Literature and Writing" theme and the "Man and the Natural World" theme. Man, in the natural world, can’t avoid being subject to time, but it is through literature, the poet argues, that he can free himself.
Even while declaring the power of prose over nature, the speaker manages to maintain the appearance of a deterministic universe.