USCIS only counts full 24 hours. The very reason for this is because people end up in in 2 places on the same 24 hour time frame. So, since a day is 24 hours, where will you put it? In both? No. If I arrive in US at 11pm, I'll put that under the foreign country. Doing that provides a fluid continuity.
I went to Mexico for less than a day. I didn't put it, per N400 instructions.
For other days, eg arrived here 4pm on 12/12/12, I put that under foreign country. 12/13/12 was my start in US, just because you can't divide the 24 hour period.