The time from a.m. to p.m. spans 12 hours.

Explanation:

• a.m. stands for "ante meridiem," meaning before noon (midnight to 11:59 a.m.).
• p.m. stands for "post meridiem," meaning after noon (12:00 p.m. to 11:59 p.m.).

For example, from 6:00 a.m. to 6:00 p.m. is exactly 12 hours.
From 12:00 a.m. (midnight) to 12:00 p.m. (noon) is also 12 hours. If you want to calculate hours between specific times, just subtract the earlier time from the later time, considering the a.m./p.m. distinction.