一日三秋

yīrìsānqiū
a single day apart seems like three seasons (idiom)

CEDICT

A single day apart seems like three years.

Wiktionary

語本《詩經．王風．采葛》：「一日不見，如三秋兮。」比喻思慕心切。《紅樓夢．第八二回》：「真真古人說：『一日三秋』，這話再不錯的。」也作「一日不見，如隔三秋」。

重編國語辭典

分別時間雖短，卻覺得很長，形容思念殷切

維基詞典

Entry Composition

1 一日 yīrì
  1. (literary or dialectal) one day
  2. (literary) another day in the future
  3. (literary) yesterday
  4. another day in the future
  5. one day
  6. yesterday
2 三 sān
  1. three
  2. 3
=3 秋 qiū
  1. autumn
  2. fall
  3. harvest time
=3 秋 qiū old variant of 秋 qiū
=3 秋 qiū used in 鞦韆 qiūqiān