(Asagao no hana hitotoki; “The morning glory flower, a single moment”)
Something’s peak or prime only lasts for a short time. Decline and entropy are all too easy. Like the morning glory flower that blooms at dawn and has already withered by midday, so is all glory fleeting, all fame transient, all things doomed. Nothing lasts, so appreciate things while they’re there.
This noun phrase begins in the middle with the noun 花 (hana), “flower.” The associative particle の (no) specifies the flowers belonging to the 朝顔 (asagao), literally “morning-face,” but actually referring to the Japanese morning glory plant. What follows seems to be a comment on this unmarked topic: number-noun 一時 (hitotoki, not “ichiji”), “one moment of time.”
Some related phrases use 槿花 (kinka), the “Korean rose” or “rose of Sharon,” whose flowers last only a single day. (Note: this is a hibiscus rather than an actual rose.)
This saying comes from the Wen Xuan (文選, Japanese Monzen), an early 6th Century CE Chinese anthology of poetry and literature, via an Edo-era zuihitsu text known as the 駿台雑話 (Sundai zatsuwa).
(“Umarete kara no hantoshi wa kawaisa kiwamari nakute asagao no hana hitotoki to iu… no ni, taihen sugite tada tada nemuritai!”)
[“They say that the half-year after birth is a beautiful fleeting moment of unlimited cuteness… but it’s too much; I just want to sleep!”]