Arriving fully upright in a country where you should dangle upside down is baffling enough. Weathermen reminding you to wear a hat in the 30-degree heat, and host families measuring your steps in meters instead of feet just add to the confusion. Get an answer of 17 o’clock when you ask for the time, and you’re likely to go into culture shock.

Of course, plenty of people get warnings about the strange thermometers and Anders Celsius, but for those travel newbies who have somehow eluded these oddities: beware. There may come a time when you have to know what 17 o’clock means on your plane ticket.

The 24-hour clock (which is called military time in the United States) is the most common time system in the world. It is used throughout Europe, and also in many African, Asian, and Latin American countries. Some places, like South Africa, actually use both the 12- and 24-hour clock. (Take a look at some of the most eye-catching clocks from around the world).

Here’s how it all works: From 1:00 AM – 12:00 PM, the 24-hour clock reads exactly the same way as our 12-hour clock. From 1:00 PM to 11:00 PM, however, the amount of 12 is added to the time. For example, 5:00 PM plus 12 is equal to 17 o’clock. To avoid confusion, midnight (12 AM) is usually just referred to as—midnight.

It’s pretty simple you say? You could have figured it out on your own without going into shock? Take another look at those complicated calculations above. Nonsense.

It turns out that all of those countries using the 24-hour clock may have the right idea. In a recent Varsity article, Cambridge University neuroscientist Dr. Michael Hastings explains that the patterns our bodies maintain—sleeping at night, being active during the day—follow a cycle that runs between 23.8 and 24.5 hours. These biochemical cycles are in turn monitored by tiny cells that produce rhythmic electrical currents. We each essentially have a miniature, ticking, 24-hour clock inside of us.