Building number confidence: Time

From the earliest of times, knowledge of the patterns of sunrise and sunset, and of the seasons, has enabled people to organise and plan their lives. For example, to gauge how much daylight is available in each new day, and when to plant and harvest crops.

Early civilisations began the development of calendars based around the:

• solar year, defined by changing seasons as the earth revolves around the sun;
• solar day, with its periods of light and darkness due to the earth rotating on its axis;
• lunar month, which follows the phases of the moon as it orbits the earth.

Various tools were also developed to aid time management, including:

• sun dials onto which shadows cast by the movement of the sun across the sky indicate the time of day;
• lunar calendars based on the monthly cycle of the Moon's phases;
• water clocks which capture the time taken for water to flow through;
• candle clocks and incense sticks which record time by the rate at which they burn.

A sundial located in Shakespeare’s Garden, Stratford upon Avon
Wikimedia / CC BY-SA 4.0

Some of these methods were more reliable than others. Consider, for example, the ability to tell the time accurately using a sundial when the sun is hidden in poor weather!

Time is measured in different units. Here are some common time measurements and how they relate to one another:

Seconds, minutes, hours and days

Days, weeks, months and years

Converting between units of time

As with other types of measurement, like length or weight, time can usually be converted into larger or smaller units using simple operations. For example:

• There are 60 seconds in a minute, so to find how many seconds there are in 3 minutes, multiply 3 by 60: 3 × 60 = 120 seconds.
• There are 60 minutes in an hour, so to find how many minutes there are in 4 hours, multiply 4 by 60: 4 × 60 = 240 minutes.
• There are 24 hours in a day, so to find how many hours there are in 3 days, multiply 3 x 24: 3 x 24 = 72 hours.
• There are 7 days in a week, so to find how many days there are in 12 weeks, multiply 12 by 7: 12 x 7 = 84 days.

Bear in mind that some conversions might require a little more consideration. For example, the number of days in 3 months may vary depending on the number of days in the months selected.

There are 24 hours in a day, however 12-hour clock time splits these into two blocks of twelve hours, which run either side of midday (noon), when the sun is at its highest point in the sky.

Analogue clock time

A standard analogue clock indicates the current time during a 12 hour period with ‘hands’ that move (clockwise) around a clockface. The small hand points to the hour (1 to 12), and the large (minute) hand indicates the number of minutes after that hour.

If the large hand points to 12, the time is 'on the hour' (0 minutes past). In this example the time is 3 o’clock.

3 o'clock

Each of the hour markers also represent a 5 minute interval (12 x 5 minutes = 60 minutes).

When the large hand is pointing to 2, for example, this indicates 10 minutes past the hour (2 x 5 = 10).

Ten minutes past 3

Some clocks also include markers for the minutes between the 5 minute intervals.

Morning, afternoon or evening?

To ensure clarity when using the 12-hour clock, we also need to know if the hour shown is during the 12 hour period:

• before midday, for example 3 o’clock in the morning or
• after midday, for example 3 o'clock in the afternoon.

We can do this using the suffix:

• am (from the Latin ante meridiem, before midday) or
• pm (post meridiem, after midday).

So

• 3 am for 3 in the morning
• 3 pm for 3 in the afternoon.

12am or 12pm?

Although midday is sometimes referred to as 12pm, and midnight as 12am, technically this is not quite right (am = 'before midday', pm = 'after midday'). To avoid any confusion, it is safer to simply say 'midday' and 'midnight' for these hours.

Royal Museums Greenwich: Is noon 12am or 12pm?

The hour markers on an analogue clock are usually displayed using the numbers from 1 to 12, or the equivalent Roman numerals.

Smaller timepieces, such as a wristwatch, may simply display markers for each of the 12 hours.

To tell the time where there are only markers, you need to remember the position of the hours. For example, 12 is always at the top of the dial and 6 is always directly opposite at the bottom.

The 24-hour clock system uses all 24 hours in the day to accurately display the time. It is not necessary to clarify whether the time stated applies to the morning, or afternoon, as this is clear from the hour shown.

Digital clock time

Whereas an analogue clock uses pointing hands which move around the clock face to indicate the current time, a digital clock displays the exact time.

The standard display of 24-hour time on a digital clock uses four digits, two either side of a colon. The first two digits display the hour (from 00 to 23*) followed by the number of minutes (from 00 to 59) past that hour.

• 00:00 to 12:00 are the (am) hours from midnight to midday/noon.
• 13:00 to 23:00 are the hours from 1pm to 11pm.

15:00 (3 pm)

Some digital clocks also display seconds, using an additional colon and a further two digits.

15:00:47 (47 seconds past 3 pm)

Stating the time

Digital (24-hour) time is spoken by stating the hour followed by the number of minutes, and the suffix ‘hrs’ (for hours) is often included.

For example:

• 19:20 (“nineteen twenty hours”)

Time on the hour (when there are two zeros in the minutes spaces) is usually spoken with the hour followed by ‘hundred’ or ‘hundred hours’.

For example:

• 15:00 (“fifteen hundred" or “fifteen hundred hours”)

The first 12 hours are the same in both systems, just written differently:

8 am = 08:00

12-hour to 24-hour time

Add 12 to afternoon (pm) hours:

3 pm = 3 +12 = 15:00 hrs

24-hour to 12-hour time:

Subtract 12 from hours greater than 12.

13:00 hrs = 13 – 12 = 1 pm

 Time check

Using 24-hour time, how would you write 7 minutes past 10 in the evening?

Toggle solution

How would you say 16:45 hrs, using the 12-hour clock system?

Toggle solution

Calculating the duration between a start and stop time

Count the number of full hours between the start and stop time, and add any minutes either side.

You may find it easier to use the 24-hour clock, particularly if start and stop times are either side of mid-day.

For example, to check the duration of a train journey, departing 10:00 and arriving at 14:00, count the number of hours on from the departure time to the arrival time: 

(10:00): 11:00, 12:00, 13:00, 14:00 (4 hours).

Click the headings below to view some examples.

Until the introduction of standardised time, local time, which was determined by the position of the sun in the sky, and with the aid of a sundial, varied from town to town and region to region across the UK.

The need for a standard time across the nation quickly became apparent with the arrival of the railways, in order to accurately schedule services transporting goods, and passengers, over long distances.

First introduced in 1840 by the Great Western Railway, Railway time was gradually adopted by all UK railway companies, with schedules organised, and station clocks aligned to London time - Greenwich Mean Time (GMT) - which was set at the Royal Observatory in Greenwich.

Glasgow Central Railway Station clock
Thomas Nugent @ Geograph / cc-by-sa/2.0

Forty years later, the Statutes (Definition of Time) Act 1880 formally defined GMT as the legal standard of time for Great Britain.

British Summer Time (BST) / Daylight Saving Time (DST)

In the UK, between the last Sunday in March and the last Sunday in October, clocks are set one hour ahead of Greenwich Mean Time (GMT) to provide an extra hour of daylight towards the end of each day.

Known as British Summer Time (BST), this practice is also in use in other countries around the world, where it is commonly referred to as Daylight Saving Time (DST).

As the earth rotates on its axis, different parts of the world are gradually exposed to the sun, bringing daylight, as others move into darkness. Over a 24 hour period, the local time of day varies around the globe as a result.

To account for these time differences, the globe has been divided into 24 time zones using imaginary lines called meridians, which run between the North and South Poles.

Most countries adopt a single time zone, even when they partially cross another meridian, however larger countries, such as the USA, must operate within a number of time zones.

World time zones map
Wikimedia / Public domain

UTC

Coordinated Universal Time, or Universal Time Coordinated (UTC), is the recognised world-wide standard for measuring clock time.

UTC+0 is the time zone at the Prime Meridian, with time in other time zones being either ahead (UTC+) or behind (UTC-).

For example, if the time in GMT/UTC-0 is 12:00 (midday), then the time in:

• UTC+2 is 2 hours ahead (14:00), and
• UTC-3 (09:00) is 4 hours behind UTC+1 (13:00).

At midday (12:00) GMT/UTC-0, the time in UTC+1 is an hour ahead (13:00), and in UTC-1 is an hour behind (11:00).

Time zone names and abbreviations

As is the custom in the UK, where we refer to Greenwich Mean Time, or GMT, other time zones around the world are also referred to by localised names and abbreviations, for example:

• UTC+1: Central European Time (CET), West Africa Time (WAT)
• UTC+9: Japan Standard Time (JST), Korea Standard Time (KST)
• UTC-5: Eastern Standard Time (EST), Columbia Time (COT).