פירוש על עבודה זרה 15:9

There is a dispute in rabbinic literature over when the world was created—Tishrei or Nisan. If the world was created in Tishrei, then Adam had never seen a long day, so we can understand why he was afraid. But if it was created in Nisan, then he’d already seen short days which grew longer and then began to grow shorter. Couldn’t he have figured out what was going on?
The (somewhat weak) answer is that he had not yet seen such short days.