Latest revision as of 07:05, 31 October 2013

The western time standards dividesthe year into 12 months, the day into 24 hours or 12 hours AM and 12 hours PM, the hour into 60 minutes and the minute into 60 seconds derived from the Babylonian^[1] mathematics using the sexagesimal^[2] or base 60 numeral system.

Note:

The base 60 systems is a highly composite number as 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30 and 60 are factors.

The length of a day

Defined in ISO 8601^[3] the day consists of 24 hours consisting of 60 minutes of 60 seconds. giving a total of <math>24 * 60 * 60 = 86.400 </math> seconds a day. With the Earth rotation slowing^[4] down there are two possible ways to make the clock time match the rotation of the Earth.

1. Decreasing the length of the second to match 86400 second/day
2. Introducing a leap second^[5] when needed.

The length of a second

Historical

The second has been used approximately since year 1000, first used by the Persian scholar Al-Biruni^[6] as the second division of 60 of the hour. From 1960 the length of second was defined as 1/86.000 of a mean solar day.

<math>\text{solar day} \eqcirc 24 \text{ hour } * \text{ }60 \text{ minute/hour } * \text{ } 60 \text{ second/minute } \eqcirc 86.400 \text{ seconds}</math>

Present day

To day the second SI-unit^[7]is defined as the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Caesium 133 atom^[8][9]. The scientific SI second used today is a little slower than the actual solar second adding to necessity of adding occasional leap seconds.

Leap second

Screencapture of the UTC clock from time.gov during the UTC leap second, on June 30, 2012, 23:59:60.

The length of a second is constant, based on the Caesium atom, so it is necessary to use leap seconds to synchronize the mean solar day to the clock time. The IERS^[10] or International Earth Rotation and Reference Systems Service introduces leap seconds at various intervals. More than 25 leap seconds has been added since 1972 by IERS^[11].

The Earth's rotation speed changes irregularly based on many factors^[12] making it necessary for IERS to introduce a leap seconds at irregular intervals adding or substracting a second giving a day with 86.401 or 86.399 seconds, depending on the Earth's rotation. So far since 1972 seconds has only been added, but events such as an earthquake may result in one or more leap seconds would be substracted.

• Adding a leap second adds the 61^st second to the minute at midnight.
  • 23:59:58 → 23:59:59: → 23:59:60 → 00:00:00
• Substracting a leap removes the 60^th second from the minute before midnight.
  • 23:59:58 → 00:00:00

Time Zones

A time zone^[13] is a region of the earth where the time clock follows the sun so that 12:00 (noon) is in midday and 00:00 (midnight) is in the night. The time zone regions often follow borders of countries for political, commercial or social purposes.

The standard clock time is defined as UTC^[14] succeeding GMT^[15].

Time zones are described as offsets from UTC. For example Denmark is UTC+1, meaning when the time clock is 10:33 in Greenwich UK it is 11:33 in Denmark. IANA maintains a list of time zones^[16].

Time zones map (Last update: December 31, 2011) - Click to enlarge

DST or summer time

Although not used by the majority of the world's countries, daylight saving time is common in the Western world.

  DST is used.

  DST is no longer used.

  DST has never been used.

Daylight saving time (DST)^[17]—also summer time in British English— is the practice of advancing clocks during the lighter months so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.

Country DST

Each country decides for itself whether to use DST or not. DST rules can be changed as each county see fit.

DST examples

Since 1981 most of the European countries has used the same date for beginning and ending European summertime.

CEST or Central European summer time

All countries in participating in CEST or Central European summertime change to summer time at the same moment, regardless of their timezone.

• CEST summer time begins (clocks go forward) at 01:00 UTC at the last Sunday in March by adding one hour to the local time, defined by the time zone.
• CEST summer time ends (clocks go backward ) at 01:00 UTC at the last Sunday in October by removing one hour from the local time.

Examples

• Denmark using time zone UTC+1^[18]
  • Denmark at time zone UTC+1 summer time begins last Sunday in March at local time 02:00, time zone then at UTC+2.
  • Denmark at time zone UTC+1 summer time ends last Sunday in October at local time 03:00, time zone back to UTC+1.
• Greenland, which spans four time zones, are using UTC-3 in all time zones, except areas around Qaanaaq, Danmarkshavn and Ittoqqortoormiit which uses UTC-4.^[19]
  • Greenland at time zone UTC-3 summer time begins last Saturday in March at local time 22:00, time zone then at UTC-2.
  • Greenland at time zone UTC-3 summer time begins last Saturday in October at local time 23:00, time zone then at UTC-3.

DST in the United States

All States and areas participating in DST change to summer time at the same moment, regardless of their timezone.^[20]

• Since 2007 summer time begins (clocks go forward) at 2:00 AM on the second Sunday of March, by adding one hour. (Note the difference in date and clock time from CEST)
• Since 2007 summer time ends (clocks go backward) at 2:00 AM on the first Sunday of November, by removing one hour. (Note the difference in date and clock time from CEST)

DST confusion

On the southern hemisphere, Australia^[21] for example, summer time begins first Sunday in October and ends the first Sunday in April, during Winter in the Northern hemisphere. Three territories in Australia have abandoned DST, adding to the confusion. Australia uses three time zones UTC+8, UTC+9:30 and UTC+10 and five time zones during summer time UTC+8, UTC+9:30, UTC+10, UTC+10:30 and UTC+11^[22]. Some territories using half hour off time zones.

New Zealand using DST from the last Sunday in September until the first Sunday in April.^[23]

It can be quite confusing to arrange International video conferences inviting participants using their local time in the invitations.

Comparing times between time zones

To compare times between countries it is necessary to know the current DST rules for each involved country to agree on a exact common time. To compare times in a historical perspective it is necessary to know the historic DST rules for each involved country. IANA^[24] tries to keep track of a historic database over DST rules^[25]

Links

• leapsecond.com (See Localtime, UTC, GPS, Loran and TAI time)

References