Tick-Tick-Tick - 24-hour Clock Format

The 24-hour clock format is a way of telling time where the day runs from midnight to midnight and is divided into $24$ hours. Unlike the 12-hour clock, it does not use 'a.m.' or 'p.m.'. Imagine a long number line starting at $0$ (midnight) and ending at $24$ (the next midnight).

In this format, time is expressed using four digits: the first two digits represent the hours and the last two digits represent the minutes. For example, $08:20$ means $8$ hours and $20$ minutes. Visualise a digital clock display where the hours always show two digits, like $05:00$ instead of just $5$.

Midnight is the start of the day and is written as $00:00$ hours. Noon is written as $12:00$ hours. If you look at a 24-hour clock face, you might see $12$ at the top, and as you move past it into the afternoon, the numbers continue as $13, 14, 15,$ and so on.

For any time from $1:00$ a.m. to $12:59$ p.m., the hours in the 24-hour format stay the same as the 12-hour format (except midnight). For example, $9:30$ a.m. becomes $09:30$ hours. You can think of this as simply adding a leading zero if the hour is a single digit.

To convert P.M. time (from $1:00$ p.m. to $11:59$ p.m.) to the 24-hour format, you add $12$ to the number of hours. For example, to find $4:00$ p.m. on a 24-hour scale, you take $4$ and add $12$ to get $16:00$ hours. This is like continuing the count after $12$ noon ($12 + 1 = 13$, $12 + 2 = 14$, etc.).

To convert a 24-hour time back to a 12-hour P.M. time, if the hour is greater than $12$, you subtract $12$ from the hours and add 'p.m.'. If the time is $20:00$, you calculate $20 - 12 = 8$, so it is $8:00$ p.m.

The 24-hour format is commonly used by railways, airlines, and the military to avoid confusion between day and night. Visualise a railway timetable where a train departing at $21:30$ clearly means $9:30$ at night, leaving no doubt for the passengers.