Issues regard­ing day­light saving have high­lighted the need for more under­stand­ing of time and how it affects busi­ness con­tinu­ity and emer­gency plan­ning managers.

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.