Q. I often check dates and times for accuracy and have come across this question multiple times. If it is midnight, does that time belong to the day that is ending or the day that is beginning? For example, if it is 11:59 p.m. on April 12, and then it turns to be 12:00 a.m., should that 12:00 a.m. be noted as being on April 12? or April 13? I’ve seen it done both ways. Thanks!
A. You’re right: We can’t really know what day 12:00 a.m. refers to. So unless it’s clear from context, it’s best to specify both, as in 12:00 a.m., April 12–13—or, better yet, midnight, April 12–13. Because even though 12:00 a.m. is usually understood to mean midnight and 12:00 p.m. noon, both of those expressions are potentially ambiguous, at least outside of calendar apps and the like (see CMOS 9.40).
The twenty-four-hour system has a slight advantage here, but you still need two separate numbers. In that system, midnight on April 12 would be expressed as 2400 hours, whereas the beginning of April 13 would be 0000 hours. Those times are identical, but the numbers used to express them—and the dates they refer to—are different. See also CMOS 9.41.