What's he saying?
"When I do count the clock that tells the time, / And see the brave day sunk in hideous night;"
When I watch time pass and night fall;
"When I behold the violet past prime, / And sable curls, all silvered o'er with white;"
When I see a violet after it has bloomed and is dying, and a woman's black hair that has become mostly white;
"When lofty trees I see barren of leaves, / Which erst from heat did canopy the herd,"
When I see the trees under which herds of animals used to take shelter now having lost their leaves,
"And summer's green all girded up in sheaves, / Borne on the bier with white and bristly beard,"
And the harvested wheat being carried away on a cart,
"Then of thy beauty do I question make, / That thou among the wastes of time must go,"
Then I question your beauty, because you, too, will age and die with time,
"Since sweets and beauties do themselves forsake / And die as fast as they see others grow;"
Because everything beautiful dies as quickly as new beauty emerges;
"And nothing 'gainst Time's scythe can make defence / Save breed, to brave him when he takes thee hence."
And there is no way to avoid the ravages of time, except by having children to succeed you when you die.
Why is he saying it?
Sonnet 12 is one of the most famous sonnets of English tradition. It is one of the "procreation" sonnets of the fair lord sequence. It directly addresses the fair lord, after contemplating the way that the passage of time exemplifies itself in nature. Though most of the poem laments the effects of time as unavoidable, the final couplet serves as a source of some hope in an otherwise wistful and resigned sonnet.
The word "brave" appears twice in this sonnet, once as an adjective describing "day" in line 2, and again in line 14 as a verb. In line 2, "brave day" is used in contrast to "hideous night," and thus it seems that "brave" is meant to imply a visual brightness and gallantry. In the last line, "brave" means to endure something without showing fear; in this case, that which much be endured is death, or time that will "take thee hence."
In the first 8 lines, the speaker describes evidence of the passage of time in nature, using imagery that hints at the comparison he is about to make to a human life. For instance, the "violet past prime" refers to a flower that has wilted and faded. Young maids were often compared to flowers; in this case, the woman has aged and is no longer fertile. The "lofty trees" that are now "barren of leaves" also reference the infertility that comes with old age, with the use of the word "barren."
Lines 7-8 reference harvested wheat being carted away, but they are also a metaphor for an old man being carried to his own funeral. "Summer's green" can be interpreted as the man, who was once young and in his prime, but now has a "white and bristly beard." The "white and bristly beard" literally refers to the whiskery growth around the grain. A "bier" is a wagon or cart, but can also mean a funeral bier, on which a coffin is carried to a funeral.
The "scythe" in line 13 is a tool with a long, curving blade, used to mow a crop. Time is often depicted wielding a scythe, with which it can cut down anything it chooses. Thus the only way to "make defence" against Time with its scythe is to "breed," preserving your youth in your progeny. This image is used in Sonnet 60, line 12: "And nothing stands but for his scythe to mow;" and in the final couplet of Sonnet 100: "Give my love fame faster than Time wastes life, / So thou prevent'st his scythe and crooked knife."