Time flies is 时光荏苒. 白驹过隙 is a term used in Chinese chess, on which I am not yet proficient enough to comment.

Time flies, you by. ~

时光荏苒- The flight of time 白驹过隙-Time passes quickly like a white pony's shadow across a crevice Describe time very fast

白驹过隙 = (a white steed flits past a crack (idiom)); How time flies!

他们应该不明白这句话是什么意思，你应该附上解释表示这是time flies