白駒過隙 [－驹过－]

Jyutping baak6 keoi1 gwo3 kwik1
Pinyin bái jū guò xì

Definitions (CC-CEDICT)
  1. a white steed flits past a crack (idiom); How time flies!
Definitions (Wiktionary)
  1. phrase, idiomatic
    swiftness of the lapse of time; fleeting of time; in the twinkling of an eye