How am I supposed to count the days on the N400? Eg if I left the US at 3 PM PT and came back the next day at 4 PM PT, does that count as 1 or 2 days?

The N-400 Application for Naturalization Form (mirror) asks for the "total days" outside the United States, for each stay outside the United States:

How am I supposed to count the days exactly on the N-400 form? E.g., if I left the US on June 20, 2020 at 3 PM PT and came back on June 21, 2020 at 4 PM PT, does that count as 1 or 2 days?

The N-400 instructions (mirror) doesn’t say how to count the days:

Part 9. Time Outside the United States

Item Number 1. Provide the total number of days (24 hours or longer)
you spent outside the United States during the last 5 years.

Item Number 2. Provide the total number of trips (24 hours or longer)
you have taken outside the United States during the last 5 years.

Item Number 3. Provide information for every trip (24 hours or longer)
you have taken outside the United States during the last 5 years.
Start with your most recent trip and work backwards.

USCIS > Policy Manual > Volume 12 – Citizenship and Naturalization > Part D – General Naturalization Requirements > Chapter 4 – Physical Presence (mirror) (thanks to user102008 for pointing to it) states:

USCIS will count the day that an applicant departs from the United States and the day he or she returns as days of physical presence within the United States for naturalization purposes. [3]

[3]: 3. USCIS will only count residence in the Commonwealth of the Northern Mariana Islands on or after November 28, 2009, as time counted for physical presence within the United States for naturalization purposes.

If I follow this rule, then leaving the US on June 20, 2020 at 3 PM PT and coming back on June 21, 2020 at 4 PM PT would count as 0 day outside the United States. I am not sure that this would please the USCIS immigration officer who will review my N-400 form.