Rabbi Eliezer says the world was created in Tishrei. Rabbi Yehoshua says the world was created in Nissan.
Surely, they are not arguing about a historical fact. How could they know historically when the world was created and what difference would it make?
Rather, they are arguing about the best way forward for Judaism following the destruction of the Temple.
Rabbi Eliezer says that it is fall. Fruit will soon rot and the days will get shorter. They must hang on to whatever they have, change nothing, and try their best to fight against the encroaching darkness.
Rabbi Yehoshua says this is spring. It is a new beginning, a time when Judaism can flourish and thrive.