Should I use tags to abbreviate a.m. and p.m.? It would seem like these are such common abbreviations that they are basically words and it would be unnecessary to use an abbreviation. However, I’m trying to be as semantic as possible. It seems that semantically it should be done.
What do you think?
A.M and P.M are initialisations of acronyms, better associated with the <acronym> element in preference to <abbr> (as their not shorthand, their just an acronym of the Latin meaning). It should also be noted that abbr is being phased out in HTML5 due to confusion between what the difference is in the English language. So basically, the most semantically correct version to use would be acronym and not abbr in that particular case, but yes it’s worth using! 
Source: http://en.wikipedia.org/wiki/Acronym_and_initialism#Early_examples_in_English
Alex, I think you got that backwards. It’s acronym going in the toilet in favor of abbr. As you say, there is confusion among web authors. As all acronyms are abbreviations, but all abbreviations are not acronyms, I think it’s just as well that acronym is deprecated in html5.
cheers,
gary
Oh sorry, my mistake, I did get it backwards, I knew one of them was being flushed 
Either way in this case I would still use acronym while it exists because it’s still the most semantically correct interpretation of AM and PM in HTML4/XHTML.
PS: Gary - I’m not a fan of HTML5 in the slightest, It’s nowhere near as well produced as XHTML2 was (I had a debate with the W3C over REV’s deprecation). 
The disadvantage of acronym is that it isn’t supported in Internet Explorer (i.e. the title attribute won’t show as a tooltip).
Now you’re the one who’s backwards Christian, IE doesn’t support ABBR in old versions prior to V8, it supports ACRONYM perfectly fine 
Personally I’d use ABBR for the ‘PM’ and ‘AM’ if I were to choose between the two tags since usually ACRONYM is pronounced as a readable word. I wouldn’t want to confuse it [AM] with “am” although obviously in the above example the old fashioned; ‘a.m.’ was used. In either case ABBR will be fine - ignoring browse bugs.
It’s getting like a tongue-twister.
Actually if you’re using that in the context of screen readers (being read out) there’s a more semantic way to fix that… aural / speech stylesheets. You re-define the way it’s pronounced by giving the ACRONYM a class value of “initial” and then place this in the appropriate CSS file with the @media aural type…
abbr, acronym {
speak: normal;
}
.initial {
speak: spell-out;
}
No point destroying semantics when it’s not necessary, ACRONYM is the correct semantic element for that particular usage.
Details, details…
I would use the 24-hour clock 
If you’re writing it as a.m. rather than am, I don’t think there’s any need to mark it as an abbreviation, in just the same way as you wouldn’t mark the abbreviations in Mr Smith or St George. People know what a.m. means, but how many of them understand ‘ante meridien’? The only advantage I can see would be to avoid ‘am’ being pronounced as the first person present tense of the verb ‘to be’ in a screen reader, but if you’re using full stops between the letters that won’t happen anyway.
Not sure that is a good idea, plenty of people don’t “get” the 24 hour clock, if you say 6PM, people know what time it is, 18:00 can be confusing to some people, you would be surprised how poor peoples comprehension of time is if they have low levels of conversion / math ability 
Do you have something to back that up? It seems rather odd that one country wouldn’t understand what the rest of the world does.
i wouldn’t bother
i mean, where do you draw the line?
<p>It's four <abbr title="of the clock">o'clock</abbr></p>
I usually do it when it’s of genuine value to explain something that needs explaining (or to give context for screen readers).
Not bothering with ABBR at all (if that’s what you’re implying Rudy) isn’t a good policy though.
I can back it up. A.m./p.m. is only used in the English speaking world (and some former British/US colonies, I’d assume). The rest of the world use ISO 8601-compliant time,
You’d especially get some problems if you need to schedule something at eg. 12:30 a.m./p.m. A lot of those who do ‘get’ the a.m./p.m. system will just add 12 to the p.m. time, and will be confused when they come across 12:30 p.m. This will then be seen as 24:30, which would be understood as ‘30 minutes past midnight the following day’.
It’d be better if the US and UK would just catch up with the rest of the world
Do your clocks show 24 hours on the hands then? Because if they only show 12 you’re running on the same AM/PM as the rest of us (and I’ve never seen a clock that goes to 24). Not saying your backwards or anything but it seems rather strange to say that we’re the only users of the AM/PM system.
Well, people around here usually know if it’s nine in the morning or nine in the evening
Also, in continental Europe, ‘half six’ means 05:30
How is that possible? Since when in any language does 5 = 6? 
‘Half six’ in continental Europe is the short form of ‘half way to six’, in stead of ‘half past six’.
So how does it make that 5:30? That’s not half way to six, half-way to six would be 3:00