atto iu ma
While saying あ
the blink of an eye; a very short time; time flies
This idiom is so common that many Japanese people use it without paying attention to its literal meaning (like “of course” in English). It is often used as あっという間に… (“in the blink of an eye”) or あっという間だった (“it was the blink of an eye”)
It stopped raining in the blink of an eye.
My high-school life was so much fun and time flew like an arrow.