This is a short tutorial covering the basics of time zones, ISO Time Format and daylight saving. As can be appreciated it is a very complex subject and there are further reading links at the end of the document.

Time Zones

A time zone is a region of the Earth. The Earth is divided into 24 zones –12 through 0 to +12. Each one is 15 degrees of latitude as measured East and West from the Prime Meridian line, which is 0 degrees latitude. Each time zone is also an hour apart as the earth rotates at 15 degrees per hour.

This line runs through the Royal Observatory in Greenwich, England. The Royal Observatory was established in 1675 amongst other things to perfect the art of navigation.

The measurement of time is fundamental to the functioning of modern society and in particular navigation and technology.

Variations in time zone do exist to take into account geographical boundaries as can be seen from the time zone map below.

There are both civilian and military designations for time zones.

The civilian ones typically use three letter abbreviations, for example EST. Military/Aviation designations use letters of the alphabet (except J, J is not found in all languages) and are known by their phonetic name. A to M are for zones East of Greenwich and N to Y are for zones West of Greenwich. Z is GMT.

These designations follow the numeric time

GMT = Greenwich Mean Time (civilian)

Z = Zulu (military)

The most commonly known means of defining time is GMT or Greenwich Mean Time, which is the mean solar time at 0 degrees latitude. GMT is also known as Universal Time

Because of variations in the Earths rotation when hyper accuracy is needed GMT cannot be used.

Coordinated Universal Time or UTC is a highly precise time scale based on atomic clocks and has uniform seconds. UTC is a compromise between the French and English variations of the term and does not stand for universal time code as is sometimes thought.

So in summary GMT is based on the Earths Rotation and UTC is based on uniform seconds as measured on highly accurate atomic clocks maintained by a number of organisations.

Whilst the difference for all but the most technically demanding applications is minor, the odd leap second, UTC should be used as the standard for time.

Network Time Protocol or NTP which is used to synchronise clocks over the internet uses UTC. NTP is a protocol designed to synchronize the clocks of computers over a network. NTP version 3 is an internet draft standard, formalized in RFC 1305. NTP version 4 is a significant revision of the NTP standard, and is the current development version, but has not been formalized in an RFC. Simple NTP (SNTP) version 4 is described in RFC 2030.

Daylight Saving Time

Daylight Saving or Summer Time is a system of advancing or retreating clocks so that a day has more daylight. Details vary by location. There are many reasons given for the reason for daylight saving from energy efficiency to people’s desire for longer summer evenings and even better voter turnout but there seems to be as many reasons for as there are against.

There also exists a great deal of variation in the implementation of daylight saving. The rules of DST also change which can cause problems for electronic or automated systems, evidenced in the recent months by the USA decision in the Energy Policy Act 2005 to move the point at which DST applies roughly 3 weeks earlier than previously.

ISO Time Format

The International Organization for Standardization is a worldwide federation of national standards bodies from some 130 countries, one from each country.

Date and Time format is defined by: ISO 8601:2000 Data elements and interchange formats — Information interchange — Representation of dates and times

The standard defines formats for numerical representation of dates, times and date/time combinations. Local time and Coordinated Universal Time (UTC) are supported.

Dates are for the Gregorian calendar (introduced in 1582), and can be given in year-month-day, year-week-day or year-day formats.

Times are given in 24hr format. All date and time formats are represented with the largest units given first, i.e., from left to right the ranking is year, month, week, day, hour, minute, second.

For example 6.36pm would be written at 18.36

Having a standardised notation system is essential for software and scientific applications.