시간 앞에 오는 전치사
in → 큰 시간 단위 (in September, in 1998, in the morning)
on → 특정 날짜/요일 (on Monday, on September 21, 1998)
at → 특정 시각/순간 (at 9 AM, at the moment)
for → 기간 (for two weeks, for a long time)
from → 시작점 (from 9 AM to 11 AM, from now on)