expression
every moment is precious; make hay while the sun shines
A classical proverb urging not to waste even a moment of time. Often used in formal or literary contexts.
「一寸の光陰軽んずべからず」という言葉を胸に刻んで勉強に励んだ。
I studied hard, keeping the proverb 'every moment is precious' in mind.
Both are proverbs about the swift passage of time, but 光陰矢の如し emphasizes speed, while 一寸の光陰軽んずべからず emphasizes the value of each moment.
A classical Chinese-derived proverb. 一寸の光陰 literally means 'one sun of light and shadow', i.e., a very short time. 軽んずべからず is an archaic negative imperative meaning 'must not make light of'.