In the United States, Canada (except Quebec), Australia, New Zealand and the Philippines, it's customary to divide the day into two 12-hour segments and to express time as either a.m. or p.m. This system introduces inaccuracies that can lead to confusion, and the military, which relies on accuracy for sensitive operations, uses a 24-hour system instead.

Converting from the civilian clock to the military time clock is easy. All you have to know is whether a 24-hour day begins at noon or at midnight. If you guessed midnight, you're right.

The division of the day into 24 hours probably goes back to the Egyptians, who were fond of dividing things into 12 parts. They didn't count by using their fingers but by using the joints on their fingers, each of which (except the thumb) has three. If you exclude the thumb, which is used to point to the joints, everybody has 12 such joints.

The Egyptians divided the day into two halves, daytime and nighttime, and allocated 12 hours for each half. The length of an Egyptian hour depended on the season, however. Fixed hours didn't become a thing until the Greeks decided they needed a reference for theoretical calculations, and Hipparchus, a Greek astronomer and mathematician, defined the hour based on the length of the day and night on the equinox. Despite this, people continued to use variable hours until mechanical clocks were invented in the 14th century.

The familiar clock face is divided into 12 sections, each of which is divided into five subsections. That's the classic 12-hour clock, and its use continues to this day. Digital clocks are quickly replacing them, however, and a digital clock can be programmed to express time in either 12- or 24-hour intervals.

When you express time in a 12-hour system, the convention is to use a.m., which means ante meridiem (before midday) and p.m., which means post meridiem (after midday). According to this terminology, midday – or noon – is the middle of the day, so the day must start at midnight. That's all you need to know to calculate military time.

The conversion is easy if you remember these simple rules:

• For times from 12:00 a.m. to 12:59 a.m., subtract 12 hours.
• For times from 1:00 a.m. to 12:59 p.m., do nothing.
• For times from 1:00 p.m. 11:59 p.m., add 12 hours.

You always express military time by specifying four digits. If the first digit is a zero, you specify that by saying "oh" or "zero." In this way, 3:30 a.m. becomes 03:30, which you would say "oh-three-thirty" or "zero three thirty." The military convention is to specify a time that's right on the hour by saying "hundred hours." For example 06:00 is "oh six hundred hours" or "zero six hundred hours."

You can always calculate military time by downloading a military time conversion app onto your mobile device, but the conversion involves only a small amount of basic arithmetic, so it's just as easy – and probably cheaper – to do it in your head. Here are a few examples to show you how easy it is:

1. Convert 12:36 a.m. to military time.

12:36 a.m. is between 12:00 a.m. and 12:59 a.m., so subtract 12 hours:

00:36 (oh oh thirty six; zero zero thirty six).

2. Convert 5:12 a.m. to military time.

5:12 is between 1:00 a.m. and 12:59 p.m., so do nothing:

05:12 (oh five twelve; zero five twelve).

3. Convert 11:00 p.m. to military time.

11:00 p.m. is between 1:00 p.m. and 11:59 p.m., so add twelve hours:

23:00 (twenty-three hundred hours).