It wasn't just a minute or two past midnight. It wasn't 12:05 or 12:10 or even 1 a.m. It was long past midnight, hours past midnight. It might have been 1:00 am, 2:00 am, 3:00 am.
If you were sitting in the living room waiting for Marmee to return, you might well have fallen asleep, and awakened when she returned. You might not know exactly what time it was, only that it was long past midnight.