The theme of this famous sonnet concerns the transcendent nature of true love and how it overcomes any barriers or obstructions. True love, the speaker argues, does not change or alter with the passing of time, or with the fading of beauty and youth. Even though love is influenced by time, love is still more powerful than time, as the following quote proves:
Love's not time's fool, though rosy lips and cheeks
Within his bending sickle's compass come;
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
Time does of course impact beauty and the appearance of love's adherents, but it does not impact love itself, as true love remains constant "even to the edge of doom," or up until death itself, and, in some cases, beyond. In many ways, therefore the poem is a pageant to the power of true love and its constancy, which also serves a double purpose: the speaker, after all, professes this kind of love for his beloved, and the more successfully and poignantly he captures the might of love the greater his chance of being heard with favourable ears by the object of his affection.