If you mean "by what means" is time portrayed, it is portrayed by the image of the trees and their flowers. If you are asking what time is portrayed as, it is portrayed as something that is fleeting and quickly gone.
Time, in this poem, seems to be measured by springs and by the flowers that come out at this time of year. The speaker talks about "springs" many times in the poem. So if you are asking what the speaker uses to mark time, it is spring.
But what is time like? Well, the main thing going on here is that time is flying and will fly by quickly. Time is something that you have to hold onto as hard as you can because it will soon be past.
Time is short, ephemeral, and transcient. The time of year is spring, when the cherry trees are filled with spring blossoms. The speaker is twenty years old, for the math he performs indicates that he has fifty springs more to live, granted that he will fill out the traditional seventy years suggested in
The days of our years are three-score years and ten; and if by reason of strength they be fourscore years, yet is their strength labor and sorrow; for it is soon cut off, and we fly away. (Psalms 90:10)
Because the speaker states that he has fifty years remaining, he has already lived twenty years of his allotted time.