AM and PM in the 12-hour clock

Allison Bloodworth abloodworth at
Wed Jan 14 16:59:02 UTC 2009

Hi Paul,

Thanks so much for this! I'd read parts of the 12-hour clock article  
but didn't notice the quote you pulled out below, a problem which I  
hadn't though about before.

Erin and I still need to talk this over in more detail, but I'm  
guessing that we'll probably leave am/pm as the selections, perhaps  
adding a notation about which one means noon and which midnight. I'm  
not sure whether that should go in the time picker itself or perhaps  
that's one case where it would be good to display it above the time  
(after it is entered) as we do with dates. I think we need AM/PM too  
because there is also the issue as to *which* midnight you are  
specifying--the beginning or end of the day? It looks like in Canada  
it could be either, so it seems midnight alone would be ambiguous. I  
think some people may know that AM is the very beginning of the day  
(from their digital clocks), but not necessarily recognize that  
midnight (in our interpretation anyway) means the same thing.

We'll get out some draft designs for this shortly--if anyone has seen  
this handled in other time pickers we'd love pointers to them! :)

Thanks again,

On Jan 13, 2009, at 1:40 PM, Paul Zablosky wrote:

> Actually, I don't think it's a common convention to regard 12:30am  
> as early afternoon, although I have come across people that believe  
> that it is so.  The point I was trying to raise is that there is  
> potential for some confusion here.  The Wikipedia article on the 12- 
> hour clock makes this clear where it mentions that:
> Those who grew up with the 24-hour clock are confused when they come  
> across situations very common in Internet forums and email, in which  
> a message indicated as posted at "12:46 am" appears before a message  
> marked "11:05 am".
> The article also points out that confusion can arise where:
> The rollover from 12 to 1 happens an hour later than the change  
> between a.m. and p.m.
> These considerations aside, I don't think this is much of an issue.   
> We can probably just stick to the convention that everything after  
> 12-noon is "pm"  and everything after 12-midnight is "am".   It's  
> just good to be aware of potential points of misunderstanding.
> The real issue is with 12:00 itself -- it's either midnight or noon,  
> but there's no universal convention for which is "am" and which is  
> "pm". The same Wikipedia article lists a number of conflicting  
> adoptions.   I'm hoping that we can find a notation that is truly  
> unambiguous and doesn't rely on a convention, but it may not be  
> easy.  One proposal is to always say "12:00 noon" or "12:00  
> midnight".  This would work fine for display, but might be a problem  
> for input.  For example: once someone selects 12:00 as the time,  
> what choices should they be offered? Should the "am/pm" choice  
> suddenly change to "noon/midnight"?  That would work, but would  
> users be comfortable with it?
> Comments?  Suggestions?
> See also Time Questions and Answers from NIST.
> Paul

Allison Bloodworth
Senior User Interaction Designer
Educational Technology Services
University of California, Berkeley
(415) 377-8243
abloodworth at

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