Time format: 12 vs. 24

Question

Which time format is most readable for users of software and websites?

• 13:00
• 1:00 PM

Answer 1

On the Conventions

First of all, it is important to note that both the display formats of the hours and the amount of hours itself in a full day (or in periods of night and day) are conventions settled*.

*^{The main source of the following information is the book "Counting Time: A Brief History of the 24-Hour Clock", which is DRM-Free and can be downloaded free of charge from your website.}

The Romans (who inherited this convention from the Egyptians and Babylonians) used the 12-hour format because they measured the period of the day apart from the 12-part period of the night, most likely due to this being the approximate number of moon cycles in a year. There was also the question that it was simply easier represent two 12-hour periods than a single with more intervals, because Roman numbers above 12 were less familiar and consumed more space in devices used for measuring time (such as a clock, for example). That is, it can be considered that since that time there were already usability concerns such as ease of understanding and learning, besides possibly some aesthetic concern.

The counting of the hours of the day in the 24-hour format emerged later (during the definition of timezones), because of the innate interpretation ambiguity of the 12-hour format. There is a "saying" that even a stopped clock is right twice a day, which comes just from this ambiguity of representation. Of course a person looking at his analog wristwatch can infer whether the time is in the morning or at night using his internal biological clock (that is, compared to how long he has been awake) and also looking out the window, but ambiguity was a major problem in defining train departure and arrival schedules.

The 12-hour textual format attempted to dim this ambiguity by adding the "am" and "pm" endings (derived from Latin "ante meridiem" and "post meridiem", that is, before noon and after noon), but still this was difficult for many people. Imagine a train ticket setting the departure time:

• 06:20am is clearly in the morning because it is 6 hours before noon, and in practice people do not spend much time considering this information.
• But 12:00pm is not equally clear. That information requires some cognitive effort to consider whether 12:00pm is noon or midnight.

Apparently, at the beginning of rail transport many people missed trips due to doubts of this kind, which initially made transport companies adopt the 24-hour format and eventually some countries as well. In addition to the book cited above, the Wikipedia has interesting information about the cultures that use one or the other format the most.

About the Usability and Experience of Each Choice

Imagine any system that uses the figure below to represent time.

This figure is easily understood by any human being as a clock with hours (small hand) and minutes (large hand), because it is a convention used for a long time in the construction of devices to measure and represent time. However, the ambiguity described above is total unless the information is about the current time (and only because it is considered that the user of the information will depend on other external information to intuit the period of the day).

Therefore, despite being a widely known metaphor (and therefore potentially useful for usability), it makes it difficult to understand if the intention is to represent the beginning of a show or film session, because the user simply will not understand the desired information (not knowing when the entertainment will start).

The most common alternative is to use a textual format as suggested in your question. For example, "06:20pm" (in 12-hour format) or "18:20" (in 24-hour format) to display the same information as the previous image. The best choice depends heavily on the cultural origin of the user (usability criterion related to social conventions) and also on some nonpragmatic preference (user experience criterion). Still, in contexts where ambiguity needs to be completely removed, we prefer to use the 24-hour format:

In most English-speaking regions, particularly in The United States, the Philippines and the United Kingdom and their former colonies, the 12-hour format is predominant, using 24-hour format in contexts in which one wishes to avoid ambiguity and have accuracy in the time tracking, such as planning public transport. (Free translation of Wikipedia)

It is interesting to note that even using the 24-hour format there may still be ambiguity regarding the day of the week in which a certain event occurs. It is common*, for example, that movie or theater sessions starting at midnight have the start time marked on the ticket as "23:59", that is, using the 24-hour format and decreasing 1 minute from real time to stay very clear for the user both the start time and the correct day on which the session will take place. It may sound silly, but this attitude is taken simply because is very important for the user experience that errors are avoided (security is another important usability criterion).

*^{I say this mainly based on my personal experience as an entertainment user and public transport in Brazil, England and France.}

Standard of Computational Representation

In the case of computational representation (internally, in reports or in the communication of data in a readable form to users), the ISO 8601 standard establishes the use of the 24-hour format because it intends that the chronological order is the same as the lexical order (for example, in the 12-hour format a lexically ordered listing would cause the time "05:00pm" to be incorrectly listed before from "06:00am" - which does not occur in 24-hour format with "17:00" and "06:00" respectively).

Concluding

There is no single answer to this question, because the best format will depend on the target audience of the system not only in relation to its cultural origin, but also personal preferences, gender and age (children may have more ease with the 12-hour format, mainly because their period of activity is usually shorter - probably the day, since at night they should sleep early, for example). In the case of fairly wide-ranging systems (operating systems are the clearest example), it is simply easier to allow configuration of the display format. However, when it is necessary to avoid ambiguity completely, as in the case of scheduling, it seems safer to use the 24-hour format.

Answer 2

Where the software is to be specified. Basing that the question is in Portuguese and to end this question.

Brazil and Portuguese use 24h instead of 12h.

Soon the best option is 13:00