Understanding Hours and Time Formats
An hour is most commonly defined as a period of time equal to 60 minutes, where a minute is equal to 60 seconds, and a second has a rigorous scientific definition. There are also 24 hours in a day. Most people read time using either a 12-hour clock or a 24-hour clock.
12-Hour Clock
A 12-hour clock uses the numbers 1-12. Depending on the clock being used, most analog clocks or watches may not include an indication of whether the time is in the morning or evening. On digital clocks and watches, "AM" stands for ante meridiem, meaning "before midday," while "PM" stands for post meridiem, or "after noon." By convention, 12 AM denotes midnight, while 12 PM denotes noon. Using the terms "12 midnight" and "12 noon" can remove ambiguity in cases where a person may not be accustomed to these conventions.
24-Hour Clock
A 24-hour clock typically uses the numbers 0-23, where 00:00 indicates midnight, and a day runs from midnight to midnight over the course of 24 hours. This time format is an international standard, and is often used to avoid the ambiguity resulting from the use of a 12-hour clock. The hours from 0-11 denote what would be the AM hours on a 12-hour clock, while hours 12-23 denote the PM hours. In certain countries, 24-hour time is referred to as military time, since this is the format used by militaries around the world, where unambiguous time measurement is critical.
Hours in Different Time Periods
The table below provides a quick reference for calculating the number of hours within standard calendar periods.
| Description | Hours |
|---|---|
| Hours in a day | 24 |
| Hours in a week | 168 |
| Hours in a month |
|
| Hours in a year |
|
| Hours in a decade |
|
| Hours in a century | 876,600 |