It's one thing to use these on output (like the Windows tray
control) where the user has many ways to infer the meaning
(most users will have some external clues about whether it is
noon or midnight),
(To exactly what context are you referring in this particular
instance? I don't see any obvious context there...)
Many computer users have windows nearby. A.m. and p.m. being
English-speaking world concepts, there are very few times and
places in English-speaking world where you can't tell the
difference between noon and midnight by looking out the window.
As the taskbar display is a clock that tells time now, user
can infer intended meaning of 12:00 a.m. or 12:00 p.m. in the
taskbar by looking out the window. Alternatively, they can wait
1 minute to see whether the next minute shows as 12:01 am or
12:01 pm. This answers the question of whether 12:00 pm is
intended to mean 1 minute before 12:01 pm or 1 minute after
11:59 pm. And if they are not that patient, most people who
have been outside or looked outside any time in the past few
days will have good enough sense of time to know whether it is
now noon or midnight.
I can't give any such context to my users when presenting them
with a screen that asks "What are you going to do and when are
you going to do it?" or "What did you do and when did you do it?"
It's really just like an airline ticket, no context -- here's where
you're going and here's when you're going. I don't believe that
any major US airline has ever intentionally printed a ticket with
a departure time of "12:00 am" or "12:00 pm."
The NIST says don't use it. I've also seen style guides from the
Software Engineering Institute and some branch of the ACM that say
don't use it. My Webster's 7th doesn't define it. The Webster's 9th
says don't use it, etc, etc. The only source that buys it is the
American Heritage Dictionary, which is about 30 degrees to the left
of whoopee, and they say not to use it when you want to avoid
ambiguity. Certainly computer software collecting user input is one
place where one should want to avoid ambiguity, as the computers don't
have any common sense to infer or verify meaning from context.
Al