I've also been known to assert this should be true for time display as well, but that's a less important nit, as in most places it is apparent from context. People have argued that this is only true for an infinitesimal period, and can be safely ignored. For a realtime display, this is arguable, as even if the display reads "12:00 PM" for a full minute, it is actually PM for most of that minute. But in a log message that occurs infrequently, it could be ambiguous.
However, when I'm entering a time into a computer (or VCR or whatever), the machine has little if any notion of context, and the machine's interpretation of "12:00 PM" has two interpretations, one wrong, and one VERY wrong. And it's not clear as to which is which.