Time flies when you’re having fun.

When having a good time, it seems like the time goes by very fast.  If time was an animal, it would be a fast bird.  Sometimes, the words underlined are said or written ‘sarcastically’, when you don’t really mean what you are saying, because it was not an enjoyable time and it seemed to go by very slowly.

[快樂時光一閃即逝。]

沒錯！美好的時光就像飛鳥、或射出的箭，倏忽間消失了蹤影！有時 Time flies 也被拿來作「反諷」用法，形容一段「難挨、不愉快」的時間。