The motif of the transience of beauty and the quick passage of time also appears in "Sonnet 12." Once again the speaker urges the young man to have children in order to preserve or save his beauty from the destructive forces of time.
In order to understand the structure of this poem, the reader should pay attention to the first word of each quatrain. Both the first and the second quatrain begin with the word when. But the third quatrain begins with the word then. The third quatrain marks the shift in the poem (beginning on line 9). And so the poem is clearly divided into an octet, which establishes the speaker's general observations about the passage of time, and a sestet, wherein the speaker comments more specifically on beauty and how it, too, fades and disappears over time.
In the first quatrain the speaker presents four images that indicate the passage of time. (1) The clock is the most obvious indicator of the passage of time. (2) But the rotation of the earth as the daylight quickly turns to night also indicates the passage of time. (3) A beautiful flower, like a violet, that has dried and withered ("past prime" meaning past its moment of beauty and splendor) also suggests time passing. However, with this third image, the speaker is also introducing the idea that something that was once beautiful will lose that beauty. And, so, the poet is, in a sense, foreshadowing the theme of the sestet.