Sonnet 12 again speaks of the sterility of bachelorhood and recommends marriage and children as a means of immortality. Additionally, the sonnet gathers the themes of Sonnets 5, 6, and 7 in a restatement of the idea of using procreation to defeat time. Sonnet 12 establishes a parallel way of measuring the passage of time, the passage of nature, and the passage of youth through life — decay. Lines 1 and 2 focus on day becoming night (the passage of time); lines 3 and 4 link nature to humankind, for the poet first evokes a flower's wilting stage (the passage of nature). Then, in line 4, the poet juxtaposes this image with black hair naturally aging and turning gray (the passage of youth) — an allusion perhaps meant to frighten the young man about turning old without having created a child. The poet then discusses the progression of the seasons, from "summer's green" to "the bier with white and bristly beard," which is an image of snow and winter. By stressing these different ways to measure time's decay, the poet hopes that the young man will finally realize that time stops for no one; the only way the young man will ensure the survival of his beauty is through offspring. This final point, that having children is the single means of gaining immortality, is most strongly stated in the sonnet's concluding couplet: "And nothing 'gainst Time's scythe can make defense / Save breed, to brave him when he takes thee hence." In these lines, "Time's scythe," a traditional image of death, is unstoppable "save breed," meaning except by having children. The fast pace of time, or the loss of it, remains a major theme in the sonnets.
Sonnet 12 is notable for its musical quality, thanks largely to the effective use of alliteration and attractive vowel runs, which are of unusual merit. This sonnet, along with Sonnet 15, which is also notable for its musical quality, is almost always included in anthologies of lyric poetry. Note the striking concluding lines and how they convey the sense of sorrow and poignancy at the thought that youth and beauty must be cut down by time's scythe. The contrast of "brave day" with "hideous night" is particularly good. And, as one critic has pointed out, the sonnets beginning with "When" are especially noteworthy because the structure of such sonnets is periodic (consisting of a series of repeated stages), making for tightness of organization, logical progression, and avoidance of a tacked-on couplet, while admirably evoking seasonal change.