Inmates is a great recomendation. Essential, must read. Terrific book.
When will it not make you put in the am? When some one takes a user perspective and changes the default to day time. And annoys the people who schedule meetings with the far east.
The point is that for people, the context of when you work is background ... generally not even recognized.
For poor dumb information systems, the context has to be surfaced and captured explicitly. So the solution is a default "normal working hours" record. PITA to configure, but can let most users just take the defaults. Perhaps even be clever and adjust as they consistently schedule meetings from 5-6 pm (after the default 5pm eod)
Ah, Nathan, but ideally software would "learn" the user preferences without the user having to explicitly save them (e.g., if all previous meetings a person scheduled are between 9am and 6pm, it should be easy to present a suggestion "am" or "pm" accordingly, that the user would only have to change in case of an exception).
Nobody would get annoyed because the customized default would reflect the person's own history, and it would be easy for the user to change the system's default when it didn't apply. A little more work on the software side, and much less work on the user side!