Answer

Oddly enough, this question seems to already have been answered (and then later removed) at StackOverflow. (this is the last existing Wayback Machine backup link to it.)

• The Unicode standard for date/time formatting calls it "period".

• Wikipedia says: The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods

• Ruby documentation (and probably a lot of strftime references) seems to refer them as "Meridiem Indicator".

From a user interface perspective, a "Period" label is too ambiguous, and "Meridiem Indicator" too pedantic, still leaving "AM/PM" as the best choice.

Aside

As for the actual meanings of AM/PM, they come from the Latin "ante meridiem" ("before noon") and "post meridiem" ("after noon").

In the film-drama context I've come across crazy board, or crazy wall.

Here's a definition from Esquire men's magazine:

Crazy Wall is the catch-all term for the boards on which investigators pin up and plot out all their clues in crime and spy thrillers, and recently it has become unthinkable for almost any serious TV drama not to have some sort of board for the characters to contemplate as they try to work out the kinks of a theory.

As @nnnnnn mentioned, there are other phrases -- investigation board or link board/chart -- used in real-life criminal investigations. Here for instance from the independent

Cops do still use link charts, but they're more helpful in complex, longitudinal investigations where there might be hundreds of people, places, phone numbers, IP addresses, etc. involved. There is now software for managing this kind of information.