The 24-hour clock can pose some difficulties if you’re not accustomed to it, which is why many people choose to use a military time conversion chart. The charts are designed to make it very simple for you to convert from the standard 12-hour clock to the 24-hour clock. The 24-hour system is generally referred to in the United States as “military time” because it is the form used by the US Armed Forces. It is also used by police officers, medical professionals, and several other professions.
Professions with overnight operations are especially reliant on the 24-hour system; these include hospitals, 24-hour restaurants, shipping firms, and many more. Some towns in northern Alaska and Canada use military time because they experience long periods without clear distinction between day and night. Millions, and perhaps billions, of people around the world use the system that might be foreign to you. To understand a conversion chart, you must first understand some elements of the 12-hour clock.
The 12-Hour Clock
The 12-hour clock is the system that most Americans learn when they’re in school and use in their daily lives. It’s the most common system in the English-speaking world and throughout most former British colonies. The system can be dated all the way back to the 1500s BCE. The ancient Mesopotamians and the ancient Egyptians both used 12-hour systems to keep time. Typically, they used a sundial during the day and a water clock at night. The ancient Romans also used a 12-hour system for daytime. Interestingly, their 12-hour system only pertained to daylight hours, and the day was always divided into twelve hours. That means that their hours were of varying lengths. Daylight hours would stretch longer during the summer and contract during the winter.
In Great Britain, the convention of the 12-hour system was preferred; it was thus transferred to the British Colonies. In the rest of Europe, however, the 24-hour system was preferred. Many historians theorize that the 24-hour system was preferred in those countries because it better mimics the motion of the earth. Also, a 24-hour clock looks similar to a sailor’s astrolabe.
To avoid confusion, the British began using Latin abbreviations to differentiate between day and night. AM stands for ante meridiem, and PM stands for post meridiem; these mean “before midday” and “after midday,” respectively.
Unlike the ancient Romans, the modern 12-hour system uses hours of standardized length, which can create confusion. For example, if someone is living in a part of northern Alaska that experiences thirty or forty days of continuous daylight, dawn and dusk look fairly similar. If you were to schedule something, you’d have to be very clear if you meant AM or PM. Furthermore, that could become confused if you were telling the time when it might be difficult to hear. That’s why the United States military began transitioning to the 24-hour system during World War I. The radios of the 1910s and 1920s did not have very clear transmission quality. With poor reception, a military operation could be as much as twelve hours off schedule because someone confused AM and PM. To avoid these confusions, the military now uses the 24-hour clock.
Converting the Time
Converting the time from the 12-hour system to the 24-hour clock is fairly simple. Before noon, you simply remove the colon from the time and add a zero. A typical reading from a 12-hour clock would say “3:00 AM.” To convert that to the 24-hour clock, you would need to remove the colon and add a zero before the three. You add the zero because the 24-hour clock always uses the same number of digits regardless of the time. The time is always displayed and read with four numbers. 3:00 AM in military time would be “0300.”
To find this on a conversion chart, you would look at the top of the chart for the time with which you’re familiar. So, you would look for the column for the 12-hour system. Then, you would move down the column until you find 3:00 AM. Then, slide to the column for military time and find the corresponding time. The chart would read “0300.” Converting in your head from AM is fairly simple. Converting from PM takes a little more work.
Since the 12-hour clock uses the same set of numbers for AM and PM, converting can be done in two different ways. If you look at the time in PM, you simply need to add twelve to the PM time. For example, if the time is 3:00 PM, you add twelve to the hours to get 1500 hours in military time. If you were to convert from military time to PM, you would subtract twelve from the military time.
For example, if the military time says that it is 1800 hours, you subtract twelve from the time to get 6:00 PM. Alternately, you can subtract two from the time and then just drop the first number. So, if the time were 2100 hours, you would subtract two from 21 to get 19. Then, drop the first number, and you have 9:00 PM.
As you can see, converting PM requires a little more work. Fortunately, you can do it very quickly with a conversion chart. In that case, just find the time in the 12-hour system column. Slide over to the 24-hour clock side to find the corresponding time.
What About Midnight?
Since the 24-hour clock constantly counts up, it must begin at zero, but military time always has four digits. So, midnight in the 12-hour system is 12:00 AM. In the 24-hour system, midnight is 0000 or 2400 hours, depending on usage. The different usages depend on the circumstance as well as the organization. You might see one or both on your conversion chart.
Most electronic correspondences or clocks use 0000 hours to denote midnight. The next minute would then be 0001 hours. The day then counts up in a predictable way. Other organizations and situations use 2400 to denote midnight. The US Navy and US Marine Corps are notable for frequently using 2400 to indicate midnight. However, members of those branches of the military, as well as other branches, often see 0000 and 2400 used interchangeably. If you see either of those on your conversion chart, you’re looking at midnight. So, why the difference?
The difference is largely one of philosophy. 2400 as midnight indicates that it is the very end of the 24-hour cycle; that means it’s the end of the day. Using 0000 as midnight indicates that it is the beginning of the 24-hour cycle and the first minute of a brand new day. Railroad times are a great example of this philosophical difference.
If you’re looking at railway arrival and departure times, you’ll often see both 0000 and 2400 used. Trains that arrive at midnight are often noted to arrive at 2400; trains that depart at midnight are often noted to depart at 0000. Since the arrival is the end of the train’s journey, the time is noted as the end of the day. If the train begins its journey at midnight, 0000 is used to indicate the beginning of the day.
If you work in the military, work with the military, or work at one of the different industries that also uses military time, you’ll likely see this for your shifts as well. A shift that begins at midnight might begin at 0000, whereas a shift that ends at midnight might end at 2400.
Who Needs to Use a Conversion Chart?
If you work in the military or an industry that uses a 24-hour clock, you likely use it for your everyday business. Those groups probably don’t need a conversion chart since they quickly become very familiar with military time. A conversion chart is most beneficial for the groups of people who have tangential relationships to users of the 24-hour clock.
One very common complaint comes from family members of military employees. The members of the Armed Forces become so familiar with the system that they use it in their everyday lives; after all, it has been proven to reduce confusion. So, the member of the military might send text messages or leave notes with military notation. Their families often use the charts to figure out the time quickly.
Furthermore, the military, the United States government, and hospitals often use contractors for their work. They might not do their own shipping and receiving, for example. Instead, they’ll work with independent firms; oftentimes, they communicate using the 24-hour system because it works best with their computers and their recordkeeping. If you are communicating with someone in the 24-hour clock, being able to quickly convert is very helpful. It’s especially helpful for large businesses that have many employees. You could post conversion charts around the office to speed up the recordkeeping.
Which Chart to Use?
There are several different charts. The one included in this article is a great example of a useful chart. You can find several other ones that provide the same information in different configurations. You should obviously use a chart that is accurate. It should also be easy to read. The purpose of a military time conversion chart is to allow you to convert between two different timekeeping systems quickly and effortlessly.