What is the poem of "on Time" by John Milton about?!

"On Time," by the English poet John Milton (1608-1674), deals with one of the most common themes in all of medieval or Renaissance literature: the theme of mutability, or the idea that life on earth is full of constant (and mainly negative) change. The inevitable passage of time was a particularly painful example of such change, especially since it ultimately involved physical deterioration and then, eventually, physical death.