12-hour clock, with a 9-hour interval between 8:30 and 5:30.
24-hour clock, 8:30 to 5:30 is 8:30 in the morning to 5:30 the next morning, 21 hours in between.
In the modern world, there are two main expressions of time: the 24-hour clock and the 12-hour clock.
The 24-hour clock, as the name suggests, is a system that divides time based on 24 hours in a day. In this system, the day starts at 00:00 (midnight) and ends at 23:59, and the time is continuous and does not repeat. One of the obvious advantages of this method of timekeeping is its high accuracy, especially in fields that require strict time management, such as military, aviation, and scientific research, where the 24-hour clock provides a more accurate representation of time.
Unlike the 24-hour clock, which is non-stop timekeeping, the 12-hour clock divides the day into two cycles, morning (am) and afternoon (pm), each containing 12 hours. Thus, one hour in the 12-hour clock occurs once in the morning and once in the afternoon. For example, 8 a.m. is denoted as 8:00 am in a 12-hour format, while 8 p.m. is denoted as 8:00 p.m. In everyday life, the 12-hour clock is widely adopted because it is easy to understand and use.
The transition between the two systems is also relatively straightforward. When switching from 12-hour to 24-hour clock, it is usually only necessary to keep the number of hours in the AM time period the same and add 12 to the hours in the PM time period (excluding 12:00 PM, which corresponds directly to 12:00 in the 24-hour format). Conversely, moving from a 24-hour clock to a 12-hour clock would require a decision to determine whether the time was in the morning or afternoon, and then decide whether to add am or pm to the number of hours.
In addition to the differences in the way they are displayed, there are also significant differences between the two systems in terms of the continuity of the temporal expression. The 24-hour clock is more suitable for the field of digitization and programming due to its continuity, as it reduces the judgment of AM and PM, simplifying the process of time calculation. In contrast, the 12-hour clock requires users to have a clear understanding of AM and PM, which adds complexity to some extent.
In practice, different countries and regions have different preferences for these two systems. Most European and Asian countries and international standards use the 24-hour system, especially in official documents, traffic signs, and radio and television broadcasts. However, in countries such as the United States, Canada, and other countries, the 12-hour clock is still very common in everyday spoken and business activities.