explain the refference _o how shall summers honey breath hold out ......................nor gates of steel so strong but Time decays?

The Sonnet is really about the ravages of time on both love and life. So "o how shall summers honey breath hold out..." could be paraphrased "How will the honeyed breath of summer withstand the ravages of time." Then he goes on to say that even strong gates of steel eventually waken as the passage of time decays it.