Wednesday, May 23, 2018

UTC Date Time System and ISO Representation

1 UTC Date Time System

1.1 UTC Time standard definition

Coordinated Universal Time (UTC) is the time standard by which the world regulates clocks and time.

The UTC is defined by an International Telecommunications Union recommendation and is based on International Atomic Time.

  • the standard adopt leap seconds to adjust UTC Time and compensate for the slowing of Earth's rotation.
  • Greenwich Mean Time (GMT) was formerly used as the international time standard, then UTC superseded GMT.
1.2 How UTC Time works

UTC divides time into days, hours, minutes and seconds.

UTC days
  • are identified using the Gregorian calendar.
  • contain 24 hours and each hour contains 60 minutes.
  • almost all UTC days contain exactly 86,400 SI seconds.
UTC minutes:
  • usually a minute contains 60 seconds, but with an occasional leap second, it may be 61 or 59.
  • because of leap seconds, the minute and all larger time units are of variable duration.
UTC seconds:
  • is the smaller time units of constant duration.
1.3 UTC Time Zones

Time zones are regions of the earth where the local time is defined as differing the UTC Time by an integer number of hours

  • but some countries have established time zones that differ by a number of half-hours or quarter-hours from UTC.
  • this difference or offset is positive or negative:
    • the westernmost time zone is twelve hours behind UTC and is UTC-12
    • the easternmost time zone is fourteen hours ahead of UTC and is UTC+14

Civil Time

Civil Time indicates the time designated by authorities or local time indicated by clocks.

  • civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), adjusted by daylight saving time during part of the year.

The Civil Time in a particular time zone can be determined from the UTC Time by adding or subtracting the number of hours and minutes specified by the UTC offset.

1.4 Daylight Saving Time

UTC does not observe daylight saving time and does not change with seasons

Local Time or Civil Time may change if a time zone jurisdiction observes Daylight Saving Time (summer time).


  • local time on the east coast of the United States is five hours behind UTC during winter, but four hours behind UTC during summer.
1.5 Who uses UTC

UTC is used by Computer systems and the Internet

  • in the internet and World Wide Web standards
  • the Network Time Protocol synchronizes the clocks of computers over the internet, using the UTC system
  • email systems and other messaging systems time-stamp messages using UTC
  • database records that include a time stamp use UTC

UTC is also the time standard used in aviation, weather forecasts, maps, amateur radio operators

2 UTC Date Time ISO Representation

ISO 8601 is an international standard that defines a method of representing dates and time in textual formats.

  • it was issued by the International Organization for Standardization (ISO).
  • it applies to representations of dates in the Gregorian calendar and times in the 24-hour notation
2.1 General principles

Date and time values are ordered from the largest to smallest unit of time: year, month (or week), day, hour, minute, second, and fraction of second.

Each date and time value has a fixed number of digits that must be padded with leading zeros.

Representations have two formats:

  • extended format using separators to enhance human readability, for example "2009-01-06"
  • basic format using a minimal number of separators, for example "20090106"
2.2 Dates

The standard organize days using the Gregorian calendar.

Calendar dates

Calendar date ISO representations are in the form:


YYYY-MM (but not YYYYMM)

  • [YYYY] indicates a four-digit year, 0000 through 9999.
  • [MM] indicates a two-digit month of the year, 01 through 12.
  • [DD] indicates a two-digit day of that month, 01 through 31.

For example, "5 April 1981" may be represented as either "1981-04-05" in the extended format or "19810405" in the basic format.

Calendar dates may be written with reduced accuracy:

  • if the day [DD] is omitted then only YYYY-MM format is allowed to avoids confusion with the truncated representation YYMMDD.

For example, one may write "1981-04" to mean "1981 April".

2.3 Times

Time ISO representations uses the 24-hour clock system and are in the form:

hh:mm:ss.sss or hhmmss.sss

hh:mm:ss or hhmmss

hh:mm or hhmm


  • [hh] indicates a zero-padded hour between 00 and 24 (where 24 is only used to denote midnight at the end of a calendar day).
  • [mm] indicates a zero-padded minute between 00 and 59.
  • [ss] indicates a zero-padded second between 00 and 60 (where 60 is only used to denote an added leap second).

For example, a time may appear either "134730" in the basic format or "13:47:30" in the extended format.

Either the seconds, or the minutes and seconds, may be omitted for greater brevity but decreased accuracy.

2.3.1 Time zone designators

Time zones are represented as local time, as UTC or as an offset from UTC.

In the local time representation, no UTC information is given.

Note that it is ambiguous to assume local time when communicating across different time zones. It is usually preferable to indicate a time zone using the standard's notation.

2.3.2 Coordinated Universal Time (UTC)

In the UTC Time representation a "Z" is added directly after the time without a separating space.

  • "Z" is the zone designator for the zero UTC offset.


  • 09:30 in the UTC Time is represented as "09:30Z" or "0930Z"
  • 14:45:15 in the UTC Time is represented as "14:45:15Z" or "144515Z".
2.3.3 Time offsets from UTC

The offsets from UTC are written in the format ±[hh]:[mm], ±[hh][mm], or ±[hh].

  • times ahead/behind of UTC have positive/negative offsets

The time zone representation is appended to the local time, in the same way of 'Z'.


  • the time zone of Berlin in the winter, which is one hour ahead of UTC, has zone designator "+01:00", "+0100" or "+01".
  • the time in New York during standard (not daylight saving) hours is UTC−05:00.

To represent a negative offset, use either a hyphen−minus or a minus sign character.

  • with Unicode character set use the minus sign
  • with ASCII character set use the hyphen−minus
  • with HTML code use the − entity


  • The following times all refer to the same moment: "18:30Z", "22:30+04", "1130−0700" and "15:00−03:30".

The offset of zero, can be expressed with the special representation "Z" or numerically as "+00:00", "+0000" or "+00".

Note: the offset from UTC changes with daylight saving time.


  • the UTC offset in Chicago is "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).
2.3.4 Time Zone Alphabetic Abbreviations

Time zones are often represented by alphabetic abbreviations such as "EST", "WST", and "CST": these abbreviations are not part of the international time and date standard and their use is not recommended.

2.4 Combined date and time representations


A point in the timeline can be represented by concatenating a date expression and a time expression using the letter T as delimiter. For example, "2018-05-20T15:25".

When using the time zone designator, it must follow the combined date and time. For example, "2018-05-20T15:25Z" or "2018-05-20T15:25+01:00".

Either basic or extended formats may be used, but both date and time must use the same format.

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