What is the central theme in the poem "What Are Years?" written by Marianne Moore?I would like to know the central theme, setting, symbolism,...

A Modernist who delights in language and precise, heartfelt expression, Marianne Moore has been compared to Picasso in his Cubist period.  For, her poems do not often adhere to any strict poetic form; she composed poetry in syllabics and "zigzag logic," (Ryan) moving swiftly from image to image.

An example of Moore's use of "zigzag logic" and swift movement from image to image, is her poem "What Are Years?"  As Kay Ryan in The Yale Review writes, "[Her] poems are glass-cased and filled with sliding doors that we open."

Therefore, to assign a central theme, the reader must follow the inner debate of Moore's poem in which there are apparent paradoxes.  For instance, one wonders how  

death, encourages others/and in its defeat, stirs/the soul to be strong?

Perhaps the answer is that the person who "accedes," or gives consent, to mortality and

in his imprisonment rises/upon himself as/the sea in a chasm, struggling to be/free and unable to be,/in its surrendering/finds its continuing

These lines recall the message of Paul Dunbar's poem "Sympathy" in which the "caged bird sings."  The bird continues to sing as he would in freedom because to not do so would surely be the end of its existence.  So, too, does the sea, locked in a chasm, find its existence in hitting the rocks. (The sea can be symbolic of a person's soul.)

The final stanza of Moore's does offer the reader some conclusion with the word so:

So, he who strongly feels,/behaves.  The very bird,/grown taller as he sings, steels/his form straight up.  Though he is captive,/his mighty singing/says,.../how pure a thing is joy./This is mortality,/this is eternity.

The final paradox of mortality=eternity is Moore's motif.  As the reader opens the "glass-doors" of the speaker's observations that in misfortune there is courage, in death's defeat there can be strength, in imprisonment there can be freedom, in captivity there can be joy; the reader realizes there is a metaphysical eternity in "behaving" to all things, and, thus, no finiteness such as "years."