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When displaying time spans(ranges, periods), do I need the AM if the time ends in the PM?

For example, is it:

9:00 - 2:00 PM or 9:00 AM - 2:00 PM?

Will the user question whether it's AM or PM if I use 9:00 - 2:00 PM?

What about those that don't cross 12:00?

9:00 - 10:00 AM or 9:00 AM - 10:00 AM

Note: these are time periods for scheduled breakout sessions on a schedule for a conference. That might play into the answer, because it's quite obvious that the 9:00 meeting doesn't start at 9:00 PM.

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Context matters. Since you mentioned that this question is about a conference schedule, take a look at this example: enter image description here

There are a few factors at play here that make this schedule easy to understand:

  • calendar lay-out: it is clear that we're talking about a time schedule here, and that the time slots are listed in their natural order
  • clean design: lots of whitespace, little visual clutter. Note that if you would add the a.m./p.m. designations at the start times as well, you would add very little information
  • content: a breakfast from 7 to 8 is in the morning; 1,5 hrs for lunch makes sense (but 13,5 doesn't)

As for choosing a 12-hr or 24-hr clock, I would follow the conventions that the majority of your audience is expecting.

Note that the schedule is inconsistent: for the Monday lunch they listed 11:30 a.m. - 1:00 p.m.; but left out the a.m. part for Tuesday and Wednesday.

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For the sake of consistency, I wouldn't add meridian extensions to just 1 time and not the others.

According to Nielsen's 10 Usability Heuristics:

Consistency and standards Users should not have to wonder whether different words, situations, or actions mean the same thing. Follow platform conventions.

It's best to add the meridian extension after ALL times so that you're 1) Consistent 2) Giving users as much information that you can

There's nothing more frustrating than looking at a time range and being unsure of whether it's AM or PM.

A related UXSE question can be found here

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    Or use a 24 hour clock, especially when you are dealing with an international audience. The am/pm use is a convention mostly bound to the US and possibly the UK commonwealth of countries, but is not used much by other nations. – Marjan Venema May 10 '14 at 11:56
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I agree with Anindya. Even if it is unlikely that anyone will misinterpret it as 9:00 pm, it's much better form to explicitly state whether it's am or pm, and that goes for any time written down (or spoken).

The alternative, of course, is to use the 24-hour clock, so 09:00 - 14:00 for 9:00 am - 2:00 pm, and 09:00 - 10:00 for 9:00 am - 10:00 am. However, if it were 10:00 - 11:00, there could be confusion, so it might be best to just use the meridian extensions in that situation.

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