time flies when you're having fun

Proverb

Time seems to pass more quickly when one is enjoying oneself.

Japanese Meaning

楽しい時は時間があっという間に過ぎる

We lost track of the afternoon at the picnic; time flies when you're having fun.

ピクニックで午後の時間を忘れてしまった。楽しいと時間が経つのはあっという間だ。

Sense（1）

Time seems to pass more quickly when one is enjoying oneself.