The atmosphere of this sonnet is somewhat melancholic. The author is pondering the fading of his youth. It is introspective and the tone, I would say, is soft. It does have hope, however, because in the end, he is hoping that he will make good use of the time he has while he is still on earth, "God willing."
The speaker admits that his youth is slipping away. He is growing old not so much on outside but more on the inside (“my semblance might deceive the truth”). He may look young on the outside, but inside him exists an old soul. He seems to be lamenting the power that “Time” has over him. He refers to “Time” as a “subtle thief” – personifying it throughout the sonnet. He comes to the realization, however, that Time is going to do what it will do and that he is no different than anyone else.
When the tone of the poem changes in this line:
Yet be it less or more, or soon or slow
the author concludes that no matter how soon or slow Time affects him, in the meantime, he will try to do God’s will (the great Task-Master). In the last lines, he acknowledges that Time may have power over him, but not outside of God’s control. Even Time is subject to the will of heaven.
Read about Milton here on enotes.