This poem is a witty example of carpe diem style--seize the day for tomorrow will be a different story!
The poem begins with the speaker praising the lady's beauty,and he tells her how much time he would spend ravishing her every inch, giving the most time to her heart since it is the most beautiful part of her. If time were unlimited, he would spend every bit of it wooing her.
The tone changes, however when he says, "BUT at my back I always hear Time's wingèd chariot hurrying near;" which tells us that he does not believe that have all the time in the world. He cannot admire her as she deserves to be loved since time is their enemy. The remainder of the poem gives us reasons why she should jump at the chance to love and be loved NOW while they are still young and beautiful.