Time is like a plant.
The year only refines and develops the seed that is born on Rosh Hashanah and that will grow over the entire year.
To borrow a metaphor from computer science, we could say that the "program" of the year is conceptualized and stored in memory on Rosh Hashanah and that the 364 other days of the year are simply spent running the program.
–Rabbi Adin Steinsaltz
From The Seven Lghts, p. 14, by Rabbi Adin Steinsaltz