At the beginning of the poem, the speaker seems rather pessimistic or fatalistic, remarking that time is finite and relentless like the tide. The collective pronoun “our” in the phrase “our minutes hasten to their end” might suggest that the speaker is writing specifically about the time he shares with a lover. He is in a melancholy mood because he realizes that the time he has together with his lover is transient.
In the next part of the sonnet, the speaker compares a human life to the rising and setting of the sun. The impression is that it is difficult to enjoy the light and warmth of the day when you understand that day is inevitably followed by the darkness and cold of night. In the same way, the speaker perhaps cannot fully enjoy the time he has with his lover because he understands that the life he shares with them will inevitably be followed by the darkness of death. The fact that the love must come to an end in the future casts a shadow over the love in the present. It is this realization which is at the heart of the speaker’s melancholy mood.
In the second half of the sonnet, time is personified, and so it becomes a character in its own right. Time is presented as ravenous and spiteful. It “confound[s]” the gift of life by taking life away. It creases “beauty’s brow” with the wrinkles of old age. It also "Feeds on the rarities of nature's truth," meaning that Time devours youthful beauty, here synonymous with truth. Time is also described as wielding a "scythe," rather like the often depicted personification of death, the Grim Reaper. In fact, the characters of Time and the Grim Reaper might be one and the same. Both take life away, indiscriminately and without mercy.
Toward the end of the poem, the speaker's melancholy tone is replaced with a somewhat more positive, defiant tone. Addressing his lover, he declares that his “verse shall stand / Praising thy worth, despite his cruel hand.” In other words, the poems that the speaker composes for his lover serve as his act of defiance against Time. Even after he and his lover are gone, having been taken by Time's “cruel hand,” their love will still survive in the form of the speaker's poems. In this way their love will defy the scythe of time.