一寸光阴一寸金，寸金难买寸光阴 yīcùnguāngyīnyīcùnjīncùnjīnnánmǎicùnguāngyīn time is precious
Literally "an inch of time is worth an inch of gold, but an inch of gold cannot buy that inch of time"; used as a proverb to emphasize that time is extremely valuable and cannot be bought back once lost.