午前 and 午後: the noon boundary and nighttime hours
Compare eleven a.m. with one p.m., and read 午前一時 without mistaking an a.m. clock label for daylight.
The clock interval before noon
午前, read ごぜん, labels clock times from midnight to before noon. 午前十一時に集合します。 schedules a gathering for eleven a.m.
午前一時に帰宅しました。 says that someone returned home at one a.m., during the night. 午前 does not necessarily describe daylight in the morning.
The clock interval from noon onward
午後, read ごご, labels clock times from noon to before midnight. 午後一時に出発します。 schedules departure for one p.m.
午後十時に電話がありました。 says that a phone call came at ten p.m. 午後 is not restricted to daylight hours.
Making noon and the date explicit
正午（しょうご）は昼の十二時です。 identifies noon as twelve in the daytime. Using 正午 makes the daytime reference explicit for a noon appointment.
Check the date as well for a 午前一時の予定. One a.m. following eleven p.m. falls on the next calendar date, so read the specified date and time together.
Does 午前一時に帰宅しました。 mean returning home in a sunlit morning?
Show the answer and why
No. It says that the person returned at one a.m., normally during the night. It is a clock label rather than a description of morning daylight.