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In developing a website for a USA audience, I'm led to believe that the 12 hr clock, with AM/PM indicators will be preferable over 24 hrs according to this Wikipedia article:

http://en.wikipedia.org/wiki/12-hour_clock#Use_by_country

Are there other considerations to take into account when it comes to web usability where people prefer the 24 hr format?

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Either way, you should include the timezone (e.g. PST, EST, etc.). –  Dan D. Nov 22 '12 at 3:31

2 Answers 2

up vote 10 down vote accepted

"Preferred" doesn't mean that 100% of your audience will be comfortable with it. Give the user the choice to change it, that way, everybody is happy.

Using 24h format also decreases ambiguity on time, specially if the clock in your website will be used to make appointments or to handle a schedule.

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Seconded. Sometimes I want one, sometimes I want the other. I would really design your system to be able to handle both. –  Rachel Keslensky Nov 22 '12 at 3:48
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I think that depending on the app, adding these sorts of options just causes preferences bloat. "Decide the little details so your customers don't have to" 37 Signals / Getting Real - obviously will depend on your target audience and your app, but making a decision and sticking with it can keep things simple. –  Jason O'Neil Nov 22 '12 at 6:00
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@JasonO'Neil: Yes, you should decide the details, and you certainly don't need to parameterize everything, but there are areas where you most certainly have to cater to preferences. Time and date formats, currency settings, basically anything in the regional settings on Windows, fall into that category. And it is not enough to detect the language/locale and adjust. Someone's preferences are not necessarily in accordance with his/her locale. Another is column widths and window sizes in desktop apps. Any app that does not honor/forgets my preferences, gets thrown off pretty darn quickly. –  Marjan Venema Nov 22 '12 at 7:11

I like the 24hr format myself and have it on my laptop and phone. But we inherently use the 12 hour mode. We never say I'm coming at 19:00 (unless you're a soldier!)

So if you can't do what Edgarator suggested and give users the choice, I say make it a 12 hour format, since the 24 hour one will make (most) users think (19-12 = 7 => 7PM), and it's our job to not make them think.

Best,

T

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Your answer assumes I'm used to thinking in 12h format. Whenever I see 5pm, I have to think ( am => ante meridiem => ante = before => 07h00 ). And about not making them think: when is 12PM, midnight or noon? –  koenmetsu Nov 22 '12 at 10:05
    
Like I said, I am a fan of the 24 hour clock too. But it is my experience from clients and even friends that the general web populace prefers the 12 hour mode. As we do not have more information about the target audience of the website, I'm inclined to think of a more average user base, and hence the preference of a 12 hour clock. If the website is targeting people like engineers or travelling management consultants, etc then perhaps a 24 hour clock would be more suited to them. Until then, I believe a 12 hour clock is better! –  Tarek Nov 22 '12 at 10:12

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