Explain the meaning of Shakespeare’s closing couplet in "Shall I compare thee to a summer's day?" How can this assertion function logically?

In Shakespeare's Sonnet 18 ("Shall I compare thee to a
summer's day"), the poet is comparing the subject of the poem with
nature.

Though the summer is a beautiful time, the object
of the poet's praise is even more "lovely and more temperate." Unfortunately, the winds
wreak havoc with the gentle blossoms that bloomed in the spring ("May"). And in the
summer, the sun can sometimes be terrible fierce ("too hot the eye of heaven shines").
He goes on to write that there is always an eventual decline in nature, with the passing
of summer. So there is the question, "Shall I compare
thee...?"

The pivotal point of the poem rests on the first
word of the ninth line: "But..."
Shakespeare summarizes an idea in the first two quatrains (four-line stanzas), but then
shifts his focus in the first line of the third quatrain. In this
case, he is saying that for all that happens in terms of nature and the summer, the
object of his praise will NOT follow the same path: "...thy eternal
summer shall not fade," even as time passes, even when death
comes.

The couplet is used to draw the sonnet to its
conclusion, but to present a summary of Shakespeare's thought: as long as there are
people left to read this sonnet, the beauty and life of this person will be
immortalized.

Whereas the beginning eight lines speak
literally of a day in summer, the third quatrain becomes more
figurative, metaphorical. The beauty of the poet's subject will not fade (literally, it
will) or be less "fair;" death will not be a threat (though
literally, it will), when this person looks towards dying and
eternity. In a literal sense, the ravages of time will leave their
mark, and death will come.

However the
rhyming couplet works because Shakespeare (or the speaker) is saying that in the sonnet,
time will not pass, and the object of the poem will be immortal, as
if he or she were frozen in time at that very
moment.