In comparing Shakespeare's sonnet (18), "Shall I compare thee to a summer's day," and Spenser's sonnet, "One day I wrote her name," there are obvious similarities.
First, both authors are speaking to someone dear; and in each of the sonnets, the author is eternalizing the subject beyond life on this earth.
In Spenser's sonnet, the speaker declares that he wrote "her" name upon the sand, and the water washed it away, alluding to the temporary nature of writing in the sand, as well as things we do in life: the water, or time, washes such things away. He wrote her name again, and again, the water washed it away.
"She" tells the author that it is vanity for men to attempt to leave a mark or eternalize anything: that all turns to dust and decay, as will she. However, he argues and says to her this is not so. Things of no importance ("baser things") may be reduced to dust, but in his verse she (and her "vertues") will be immortalized; her "glorious" name will be scrawled across the heavens. The last two lines, the rhyming couplet, conclude or summarize the intent of his sonnet. The speaker says that when all the world is subdued by death, their love will live on, and be renewed—eternalized through these verses.
Likewise, in Shakespeare's sonnet, he also speaks of the temporary condition of worldly things, using images of nature. The speaker asks if he should compare the beauty of his subject to a summer day? But he admits that "thou" is lovlier and more "comfortable" ("temperate") than a summer's day which can be hot and uncomfortable. Symbolically, when he refers to the rough winds shaking the "darling buds of May," the speaker is referring to how the passing of time can rob one of the beauty of youth ("May" or spring), that summer days are gone too quickly, that sometimes the sun is just too hot to bear or after at time, its brilliance dims—like beauty. The author remarks, also, that beauty will in time fade ("decline") either by things that wear one down in life or by nature, the aging that is natural to all things.
There is, however, a shift at the beginning of line nine, which is a trademark of Shakespeare's sonnets. It is almost as if he says "except" or "but." The attention moves along a different path now, leaving the description of nature fading and beauty diminishing. The speaker goes on to say that the subject of the poem need not worry about growing old ("thy eternal summer shall not fade"), or losing any of the loveliness that resides there already. Death will not come like a thief to rob her, overshadowing her loveliness by taking her life. "When in eternal lines to time thou growest" means that he will make her immortal by writing these lines to describe her. The rhyming couplet at the end (the last two lines) work as do those of Spenser: the speaker summarizes the intent of the sonnet. He claims that as long as men breathe, or can see and can read this sonnet, it will give her life—and her beauty will never fade, but live on forever.
The similarities of these sonnets both surround preserving the personage and beauty of the woman who is the subject of the poem, thus alluding, also, to the power of words that are equally as powerful as the subject which each speaker praises and perpetuates.