Even by the standards of the Shakespearean sonnet, this poem is very neatly divided into three self-contained quatrains and a couplet. The quatrains provide three complementary views of the passage of time. In the first, the minutes pass like waves over pebbles, aggressively thrusting themselves forward. In the second, the pace of the lines slows, with majestic Latinate words (e.g., "nativity," "maturity") giving an internal rhyme that balances one against the other. Time is personified, almost deified, as the power that gives and takes away.
In the third quatrain, a more explicitly destructive personified Time scores lines in the brow, feeds upon and mows down his victims—that is, everyone, but not, perhaps, everything. As the sonnet turns into the final couplet, the poet hopes that his verse, praising his beloved, will be spared the ravages of time.
This is perhaps Shakespeare's favorite subject, more famously treated in such sonnets as Sonnet 18 and Sonnet 55, but it is here expressed with great precision and perfection.