Shakespeare's Sonnet 18 comes close to being an extended simile, without ever quite being one. A poem which said "You are like a summer's day, in the following ways" would clearly be a simile on the same expansive scale as Homer's comparisons. Here, however, the poet cannot make up his mind that his beloved should be compared to a summer's day, then proceeds to list the ways in which the two are dissimilar.

There are several metaphors within the poem, however. In the fourth line, the image is drawn from the legal profession. Summer is described as being like the tenancy of a house with a short-term lease, emphasizing the brevity of the season. In the following two lines, the sun is first described as an eye, then as a face which has a "gold complexion." Both these metaphors personify the sun, an effect reinforced by the use of the pronoun "he."

At the end of the second quatrain, the changing of the seasons is compared to a ship changing course with "untrimm'd" sails. Finally, there is the personification of death in the third quatrain. Personification is a type of metaphor, since the object (or, in this case, the concept) is being described as a human being, in this case a boastful one, who is inclined to brag about the extent of his power.

The overall conceit, or extended metaphor, of the sonnet is that the speaker finds his beloved superior to a summer's day. In comparing him, he says that he is even lovelier and "more temperate." The summer days to which he compares him have "all too short a date," meaning their beauty is temporary when contrasted with his enduring beauty.

To deepen the metaphor of the "temperate" nature of his beauty, the speaker points out that the summer sun is sometimes "too hot" and sometimes does not shine at all ("his gold complexion dimm'd"), but that his, metaphoric, "eternal summer shall not fade."

As the sonnet concludes, the speaker assures his beloved that his beauty will be immortal because it will, metaphorically speaking, live forever because whenever a person reads the sonnet, he will live again. Even death will not be able to overcome his radiance and place him "in his shade."

The speaker begins the poem by comparing his lover to a summer day, claiming that he is, ultimately, more beautiful and more pleasantly moderate: sometimes the winds blow roughly and summer isn't really around for very long anyway.  One might argue that, though this is a comparison, it isn't really a metaphor because the speaker doesn't say that one thing is another: he merely asks if it is worthwhile to compare the two things and then explains that it probably is because his lover is better.

As further proof that his lover is even better than a summer day, the speaker continues, saying that the sun sometimes shines too hotly and the beauty of things fades over time.  He calls the sun "the eye of heaven," employing a metaphor which compares the sun to an eye.  However, his lover's "eternal summer" will never go away, as the real summer does.  Here, he compares his beauty, via metaphor, to an everlasting summer in order to emphasize his belief that it will never fade. 

The speaker also says that death (which is personified) will never be able to brag that this beautiful lover "wander[s] in his shade."  One could argue that he employs another metaphor here to compare the experience of being dead to "wander[ing] in [...] shade."  

Similes and metaphors are all about comparisons, and Shakespeare begins his poem by asking whether he should compare the object of his affection to a summer day. Metaphor is the basis of Sonnet 18, but no simile ever appears.

The entire poem is built around a metaphor. He compares his love to a summer day, but then goes on to explain what makes him better than even a lovely day. He says:

• He is more lovely, with no rough winds.
• He is more temperate, with no hot sun.
• He doesn't have cloudy days.

He goes on to say that beautiful things stop being beautiful. However, his love is an eternal summer that will never fade. He won't lose his beauty.

Shakespeare ends the poem by explaining that his love will never really die because he'll always be remembered in this poem. 

A simile is the comparison between two things using the words "like" or "as" to denote the comparison. A metaphor, on the other hand, does not use the words "like" or "as" to denote the comparison.

In Shakespeare's Sonnet 18, "Shall I Compare the to a Summer's Day," there are multiple metaphors.

1. The first metaphor appears in line one. The metaphor is the comparison between the subject of the poem to a summer's day.

Shall I compare thee to a summer's day?

2. The next metaphor appears in line five. Here, the comparison is made between the sun and a hot eye.

Sometime too hot the eye of heaven shines.

3. One final metaphor is found in line eight. The seasons of the year are compared to a change of course.

By chance or nature's changing course untrimm'd.

No similes appear in Sonnet 18.