The white colt in the white colt passage refers to a white pony, Bai refers to white, and Ju refers to a small pony. The expression "white horse passing through a gap" is a metaphor for time being like a little white horse running through a small gap. Describe time passing very quickly.
The passage of time refers to the astonishingly fast passing of time. It can be understood that Zhuangzi believed that life is short, but some people think that Zhuangzi attained enlightenment while sitting in meditation and sighed when he saw the shortness of ordinary people's lives.
The white colt passing by comes from a passage in "Zhuangzi·Waipian·Zhibeiyou": Between heaven and earth in life, if a white colt passes by, it happens suddenly. If the note is full of vitality, it will always come out; if it is oily and detached, it will never fail to enter. It has transformed into life, and transformed into death. Creatures grieve, humans grieve. Unravel the heavenly tao and fall into the heavenly bow. It's so chaotic that the soul will pass away, but the body will follow it. It's a great return.