一日三秋

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

CEDICT

A single day apart seems like three years.

Wiktionary

语本《诗经．王风．采葛》：「一日不见，如三秋兮。」比喻思慕心切。《红楼梦．第八二回》：「真真古人说：『一日三秋』，这话再不错的。」也作「一日不见，如隔三秋」。

重編國語辭典

分别时间虽短，却觉得很长，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 秋千 qiūqiān