on at in by about time

on 用在日或者和日连在一起的一个时间段 :on Sunday ,on Sunday afternoon

at用在时间点,一瞬间,或者节假日(可能应为节假日过的很快)at noon,at 9:00 am ,at the moment,at  Spring Festival

by 最迟在XXX之前,到XXX已经 i will arrive here by 12:00 am,I was very tired by 12 am

in接一个时间段,通常是大时间段, in 1990s  ,in Jan. 表示在多长时间之里,或者加the 的时间如i n the morning

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