ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization (ISO) and was first published in 1988. The purpose of this standard is to provide an unambiguous and well-defined method of representing dates and times, so as to avoid misinterpretation of numeric representations of dates and times, particularly when data are transferred between countries with different conventions for writing numeric dates and times.
In general, ISO 8601 applies to representations and formats of dates in the Gregorian (and potentially proleptic Gregorian) calendar, times based on the 24-hour timekeeping system (including optional time zone information), time intervals and combinations thereof. The standard does not assign any specific meaning to elements of the date/time to be represented; the meaning will depend on the context of its use. In addition, dates and times to be represented cannot not include words with no specified numerical meaning in the standard (e.g. names of years in the Chinese calendar) or that do not use characters (e.g. images, sounds).
In representations for interchange, dates and times are arranged so the largest temporal term (the year) is placed to the left and each successively smaller term is placed to the right of the previous term. Representations must be written in a combination of Arabic numerals and certain characters (such as "-", "W", and "Z") that are given specific meanings within the standard; The implication is that some commonplace ways of writing parts of dates, such as "January" or "Thursday", are not allowed in interchange representations.
|Date and time (current at page generation)
expressed according to ISO 8601:
|Combined date and time in UTC:||2014-08-20T19:23:25+00:00
|Date with week number:||2014-W34-3|
- 1 History
- 2 General principles
- 3 Dates
- 4 Times
- 5 Combined date and time representations
- 6 Durations
- 7 Time intervals
- 8 Truncated representations
- 9 Usage
- 10 Related standards
- 11 See also
- 12 Notes and references
- 13 External links
The first edition of the ISO 8601 standard was published in 1988. It unified and replaced a number of older ISO standards on various aspects of date and time notation: ISO 2014, ISO 2015, ISO 2711, ISO 3307, and ISO 4031. It has been superseded by a second edition in 2000 and by the current third edition published on 3 December 2004. ISO 8601 was prepared by, and is under the direct responsibility of, ISO Technical Committee TC 154.
ISO 2014, though superseded, is the standard that originally introduced the all-numeric date notation in most-to-least-significant order [YYYY]-[MM]-[DD]. The ISO week numbering system was introduced in ISO 2015, and the identification of days by ordinal dates was originally defined in ISO 2711.
- Date and time values are ordered from the most to the least significant: year, month (or week), day, hour, minute, second, and fraction of second. The lexicographical order of the representation thus corresponds to chronological order, except for date representations involving negative years. This allows dates to be naturally sorted by, for example, file systems.
- Each date and time value has a fixed number of digits that must be padded with leading zeros.
- Representations can be done in one of two formats – a basic format with a minimal number of separators or an extended format with separators added to enhance human readability. The standard notes that "The basic format should be avoided in plain text." The separator used between date values (year, month, week, and day) is the hyphen, while the colon is used as the separator between time values (hours, minutes, and seconds). For example, the 6th day of the 1st month of the year 2009 may be written as "2009-01-06" in the extended format or simply as "20090106" in the basic format without ambiguity.
- For reduced precision, any number of values may be dropped from any of the date and time representations, but in the order from the least to the most significant. For example, "2004-05" is a valid ISO 8601 date, which indicates May (the fifth month) 2004. This format will never represent the 5th day of an unspecified month in 2004, nor will it represent a time-span extending from 2004 into 2005.
- If necessary for a particular application, the standard supports the addition of a decimal fraction to the smallest time value in the representation.
|This section's factual accuracy is disputed. (August 2014)|
||This section possibly contains original research. (August 2014)|
The standard delineates Gregorian calendar to mean a, possibly infinite, time scale of adjoining calendar years that has a reference point as well as several properties and limits. The properties include continuous/sequential numbering of years consisting of 365 or 366 days, 52 or 53 weeks, 12 months with stipulated names and numbers of days in each, and stipulated named days of the week. The limits, which can be overridden by mutual agreement between parties, include a lower bound calendar year of 1583 and an upper bound calendar year of 9999. These rules and the term "Gregorian calendar" are specifically differentiated from the traditional Gregorian calendar, which it defined as the "calendar in general use, introduced in 1582 to define a calendar year that more closely approximated the tropical year than the Julian calendar". References to the term "Gregorian calendar" found in this article should be interpreted as meaning the definition set out and not the traditional Gregorian calendar unless otherwise specified.
The standard reference point has been set at 1875-05-20, the signing date of the Convention du Mètre (Metre Convention) in Paris. Calendar dates prior to 1875-05-20 are still compatible with the standard back to 1583, with the qualification that they conform to the rules for Gregorian calendars specified in section 3.2.1. Dates prior to 1583 use the proleptic Gregorian calendar and should only be used by mutual agreement. The propleptic reference point year is "0000", which has 366 days.
The standard does not forbid conversion of incompatible calendars to conform with the specifications therein. Conversions must not be used for exchange purposes unless mutually agreed to. If mutual agreement exists for conversion, the standard strongly recommends the use of Gregorian UTC in general, though does not make any requirements. Once conformed to the rules of the standard it can then be formatted as specified by the standard and the agreement.
ISO 8601 prescribes, as a minimum, a four-digit year [YYYY] to avoid the year 2000 problem. It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD. However, years prior to 1583 are not automatically allowed by the standard. Instead "values in the range  through  shall only be used by mutual agreement of the partners in information interchange."
To represent years before 0000 or after 9999, the standard also permits the expansion of the year representation but only by prior agreement between the sender and the receiver. An expanded year representation [±YYYYY] must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign instead of the common AD or BC notation; by convention 1 BC is labelled +0000, 2 BC is labeled -0001, and so on.
|YYYY-MM||(but not YYYYMM)|
Calendar date representations are in the form shown in the box to the right. [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.
The standard also allows for calendar dates to be written with reduced precision. For example, one may write "1981-04" to mean "1981 April", and one may simply write "1981" to refer to that year or "19" to refer to the century from 1900 to 1999 inclusive. Although the standard allows both the YYYY-MM-DD and YYYYMMDD formats for complete calendar date representations, if the day [DD] is omitted then only the YYYY-MM format is allowed. By disallowing dates of the form YYYYMM, the standard avoids confusion with the truncated representation YYMMDD (still often used).
Week date representations are in the format as shown in the box to the right. [YYYY] indicates the ISO week-numbering year which is slightly different from the traditional Gregorian calendar year (see below). [Www] is the week number prefixed by the letter W, from W01 through W53. [D] is the weekday number, from 1 through 7, beginning with Monday and ending with Sunday. This form is popular in the manufacturing industries.
There are mutually equivalent descriptions of week 01:
- the week with the year's first Thursday in it (the formal ISO definition),
- the week with 4 January in it,
- the first week with the majority (four or more) of its days in the starting year, and
- the week starting with the Monday in the period 29 December – 4 January.
If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year (there is no week 00). 28 December is always in the last week of its year.
The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday of the year.
The ISO week-numbering year starts at the first day (Monday) of week 01 and ends at the Sunday before the new ISO year (hence without overlap or gap). It consists of 52 or 53 full weeks. The ISO week-numbering year number deviates from the number of the traditional Gregorian calendar year on a Friday, Saturday, and Sunday, or a Saturday and Sunday, or just a Sunday, at the start of the traditional Gregorian calendar year (which are at the end of the previous ISO week-numbering year) and a Monday, Tuesday and Wednesday, or a Monday and Tuesday, or just a Monday, at the end of the traditional Gregorian calendar year (which are in week 01 of the next ISO week-numbering year). For Thursdays, the ISO week-numbering year number is always equal to the traditional Gregorian calendar year number.
- Monday 29 December 2008 is written "2009-W01-1"
- Sunday 3 January 2010 is written "2009-W53-7"
For an overview of week numbering systems see week number. The US system has weeks from Sunday through Saturday, and partial weeks at the beginning and the end of the year. An advantage is that no separate year numbering like the ISO week-numbering year is needed, while correspondence of lexicographical order and chronological order is preserved.
An ordinal date is a simple form for occasions when the arbitrary nature of week and month definitions are more of an impediment than an aid, for instance, when comparing dates from different calendars. As represented above, [YYYY] indicates a year. [DDD] is the day of that year, from 001 through 365 (366 in leap years). For example, "1981-04-05" is also "1981-095".
This format is used with simple hardware systems that have a need for a date system, but where including full calendar calculation software may be a significant nuisance. This system is sometimes incorrectly referred to as "Julian Date", whereas the astronomical Julian Date is a sequential count of the number of days since day 0 beginning 1 January 4713 BC Greenwich noon, Julian proleptic calendar (or noon on ISO date -4713-11-24 which uses the Gregorian proleptic calendar with a year ).
ISO 8601 uses the 24-hour clock system. The basic format is [hh][mm][ss] and the extended format is [hh]:[mm]:[ss].
- [hh] refers to a zero-padded hour between 00 and 24 (where 24 is only used to denote midnight at the end of a calendar day).
- [mm] refers to a zero-padded minute between 00 and 59.
- [ss] refers to a zero-padded second between 00 and 60 (where 60 is only used to denote an added leap second).
So a time might appear as either "134730" in the basic format or "13:47:30" in the extended format.
Midnight is a special case and may be referred to as either "00:00" or "24:00". The notation "00:00" is used at the beginning of a calendar day and is the more frequently used. At the end of a day use "24:00". "2007-04-05T24:00" is the same instant as "2007-04-06T00:00" (see Combined date and time representations below).
Decimal fractions may be added to any of the three time elements. However, a fraction may only be added to the lowest order time element in the representation. A decimal mark, either a comma or a dot (without any preference as stated in resolution 10 of the 22nd General Conference CGPM in 2003, but with a preference for a comma according to ISO 8601:2004) is used as a separator between the time element and its fraction. To denote "14 hours, 30 and one half minutes", do not include a seconds figure. Represent it as "14:30,5", "1430,5", "14:30.5", or "1430.5". There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties.
Time zone designators
If no UTC relation information is given with a time representation, the time is assumed to be in local time. While it may be safe to assume local time when communicating in the same time zone, it is ambiguous when used in communicating across different time zones. It is usually preferable to indicate a time zone (zone designator) using the standard's notation.
If the time is in UTC, add a Z directly after the time without a space. Z is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".
UTC time is also known as 'Zulu' time, since 'Zulu' is the NATO phonetic alphabet word for 'Z'.
Time offsets from UTC
||This article may be confusing or unclear to readers. (October 2013)|
The offset from UTC is given in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, would be "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).
The following times all refer to the same moment: "18:30Z", "22:30+04", "1130−0700", and "15:00−03:30". Nautical time zone letters are not used with the exception of Z. To calculate UTC time one has to subtract the offset from the local time, e.g. for "15:00−03:30" do 15:00 − (−03:30) to get 18:30 UTC.
An offset of zero, in addition to having the special representation "Z", can also be stated numerically as "+00:00", "+0000", or "+00". However, it is not permitted to state it numerically with a negative sign, as "−00:00", "−0000", or "−00". The section dictating sign usage (section 3.4.2 in the 2004 edition of the standard) states that a plus sign must be used for a positive or zero value, and a minus sign for a negative value. Contrary to this rule, RFC 3339, which is otherwise a profile of ISO 8601, permits the use of "−00", with the same denotation as "+00" but a differing connotation.
Combined date and time representations
A single point in time can be represented by concatenating a complete date expression, the letter T as a delimiter, and a valid time expression. For example "2007-04-05T14:30".
If a time zone designator is required, it follows the combined date and time. For example "2007-04-05T14:30Z" or "2007-04-05T12:30-02:00".
Either basic or extended formats may be used, but both date and time must use the same format. The date expression may be calendar, week, or ordinal, and must use a complete representation. The time expression may use reduced accuracy. It is permitted to omit the 'T' character by mutual agreement.
Durations are a component of time intervals and define the amount of intervening time in a time interval. They should only be used as part of a time interval as prescribed by the standard. Time intervals are discussed in the next section.
Durations are represented by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S or P[n]W as shown to the right. In these representations, the [n] is replaced by the value for each of the date and time elements that follow the [n]. Leading zeros are not required, but the maximum number of digits for each element should be agreed to by the communicating parties. The capital letters P, Y, M, W, D, T, H, M, and S are designators for each of the date and time elements and are not replaced.
- P is the duration designator (historically called "period") placed at the start of the duration representation.
- Y is the year designator that follows the value for the number of years.
- M is the month designator that follows the value for the number of months.
- W is the week designator that follows the value for the number of weeks.
- D is the day designator that follows the value for the number of days.
- T is the time designator that precedes the time components of the representation.
- H is the hour designator that follows the value for the number of hours.
- M is the minute designator that follows the value for the number of minutes.
- S is the second designator that follows the value for the number of seconds.
For example, "P3Y6M4DT12H30M5S" represents a duration of "three years, six months, four days, twelve hours, thirty minutes, and five seconds".
Date and time elements including their designator may be omitted if their value is zero, and lower order elements may also be omitted for reduced precision. For example, "P23DT23H" and "P4Y" are both acceptable duration representations.
To resolve ambiguity, "P1M" is a one-month duration and "PT1M" is a one-minute duration (note the time designator, T, that precedes the time value). The smallest value used may also have a decimal fraction, as in "P0.5Y" to indicate half a year. This decimal fraction may be specified with either a comma or a full stop, as in "P0,5Y" or "P0.5Y". The standard does not prohibit date and time values in a duration representation from exceeding their "carry over points" except as noted below. Thus, "PT36H" could be used as well as "P1DT12H" for representing the same duration.
Alternatively, a format for duration based on combined date and time representations may be used by agreement between the communicating parties either in the basic format PYYYYMMDDThhmmss or in the extended format P[YYYY]-[MM]-[DD]T[hh]:[mm]:[ss]. For example, the first duration shown above would be "P0003-06-04T12:30:05". However, individual date and time values cannot exceed their moduli (e.g. a value of 13 for the month or 25 for the hour would not be permissible).
A time interval is the intervening time between two time points. The amount of intervening time is expressed by a duration (as described in the previous section). The two time points (start and end) are expressed by either a combined date and time representation or just a date representation.
There are four ways to express a time interval:
- Start and end, such as "2007-03-01T13:00:00Z/2008-05-11T15:30:00Z"
- Start and duration, such as "2007-03-01T13:00:00Z/P1Y2M10DT2H30M"
- Duration and end, such as "P1Y2M10DT2H30M/2008-05-11T15:30:00Z"
- Duration only, such as "P1Y2M10DT2H30M", with additional context information
Of these, the first three require two values separated by an interval designator which is usually a solidus or forward slash "/". Section 4.4.2 of the standard notes that: "In certain application areas a double hyphen is used as a separator instead of a solidus." The standard does not define the term "double hyphen", but previous versions used notations like "2000--2002". Use of a double hyphen instead of a solidus allows inclusion in computer filenames. A solidus is a reserved character and not allowed in a filename in common operating systems.
For <start>/<end> expressions, if any elements are missing from the end value, they are assumed to be the same as for the start value including the time zone. This feature of the standard allows for concise representations of time intervals. For example, the date of a two-hour meeting including the start and finish times could be simply shown as "2007-12-14T13:30/15:30", where "/15:30" implies "/2007-12-14T15:30" (the same date as the start), or the beginning and end dates of a monthly billing period as "2008-02-15/03-14", where "/03-14" implies "/2008-03-14" (the same year as the start).
If greater precision is desirable to represent the time interval, then more time elements can be added to the representation. An interval denoted "2007-11-13/15" can start at any time on 2007-11-13 and end at any time on 2007-11-15, whereas "2007-11-13T09:00/15T17:00" includes the start and end times. To explicitly include all of the start and end dates, the interval would be represented as "2007-11-13T00:00/15T24:00".
Repeating intervals are specified in clause "4.5 Recurring time interval". They are formed by adding "R[n]/" to the beginning of an interval expression, where R is used as the letter itself and [n] is replaced by the number of repetitions. Leaving out the value for [n] means an unbounded number of repetitions. If the interval specifies the start (forms 1 and 2 above), then this is the start of the repeating interval. If the interval specifies the end but not the start (form 3 above), then this is the end of the repeating interval. For example, to repeat the interval of "P1Y2M10DT2H30M" five times starting at "2008-03-01T13:00:00Z", use "R5/2008-03-01T13:00:00Z/P1Y2M10DT2H30M".
ISO 8601:2000 allowed truncation (by agreement), where leading components of a date or time are omitted. Notably, this allowed two-digit years to be used and the ambiguous formats YY-MM-DD and YYMMDD. This provision was removed in ISO 8601:2004.
On the Internet, the World Wide Web Consortium (W3C) uses ISO 8601 in defining a profile of the standard that restricts the supported date and time formats to reduce the chance of error and the complexity of software.
RFC 3339 defines a profile of ISO 8601 for use in Internet protocols and standards. It explicitly excludes durations and dates before the common era. The more complex formats such as week numbers and ordinal days are not permitted.
RFC 3339 deviates from ISO 8601 in allowing a zero timezone offset to be specified as "-00:00", which ISO 8601 forbids. RFC 3339 intends "-00:00" to carry the connotation that it is not stating a preferred timezone, whereas the conforming "+00:00" or any non-zero offset connotes that the offset being used is preferred. This convention regarding "-00:00" is derived from earlier RFCs, such as RFC 2822 which uses it for timestamps in email headers. RFC 2822 made no claim that any part of its timestamp format conforms to ISO 8601, and so was free to use this convention without conflict. RFC 3339 errs in adopting this convention while also claiming conformance to ISO 8601.
ISO 8601 is referenced by several specifications, but the full range of options of ISO 8601 is not always used. For example, the various electronic program guide standards for TV, digital radio, etc. use several forms to describe points in time and durations. The ID3 audio meta-data specification also makes use of a subset of ISO 8601. The GeneralizedTime makes use of another subset of ISO 8601.
The ISO 8601 week date, as of 2006, appeared in its basic form on major brand commercial packaging in the United States. Its appearance depended on the particular packaging, canning, or bottling plant more than any particular brand. The format is particularly useful for quality assurance, so that production errors can be readily traced to work weeks, and products can be correctly targeted for recall.
|Australia||AS ISO 8601-2007|
|Austria||OENORM EN 28601|
|Belgium||NBN EN 28601|
|Czech Republic||ČSN ISO 8601|
|European Norm||EN 28601:1992|
|France||NF Z69-200; NF EN 28601:1993-06-01|
|Germany||DIN ISO 8601:2006-09 (replaced DIN EN 28601:1993-02); related: DIN 5008:2011-04|
|Greece||ELOT EN 28601|
|Hungary||MSZ ISO 8601:2003|
|Iceland||IST EN 28601:1992|
|Italy||UNI EN 28601|
|Japan||JIS X 0301-2002|
|Lithuania||LST ISO 8601:1997|
|Netherlands||NEN ISO 8601 & NEN EN 28601 & NEN 2772|
|People's Republic of China||GB/T 7408-2005|
|Portugal||NP EN 28601|
|Russia||ГОСТ ИСО 8601-2001 (current), ГОСТ 7.64-90 (obsolete)|
|South Africa||ARP 010:1989|
|Spain||UNE EN 28601|
|Switzerland||SN ISO 8601:2005-08|
|Thailand||TIS 1111:2535 in 1992|
|Turkey||TS ISO 8601|
|UK||BS ISO 8601:2004, BS EN 28601 (1989-06-30)|
|Ukraine||ДСТУ ISO 8601:2010|
|US||ANSI INCITS 30-1997 (R2008) and NIST FIPS PUB 4-2|
Media related to ISO 8601 at Wikimedia Commons
Notes and references
- ISO 8601:2004[E] section 1 Scope
- ISO 8601:2004(E), ISO, 2004-12-01, "Annex A ... From that concept representations of all other date and time values were logically derived; thus, ISO 2014, ISO 3307 and ISO 4031 have been superseded.... Identification of a particular date by means of ordinal dates (ISO 2711) and by means of the week numbering system (ISO 2015) were alternative methods that the basic concept of this International Standard could also encompass; thus, ISO 2015 and ISO 2711 have now been superseded."
- ISO 8601:2004(E). ISO. 2004-12-01. p. iv Foreword.
- TC 154 Processes, data elements and documents in commerce, industry and administration, Technical committees – ISO
- ISO, FAQ: Numeric representation of Dates and Time
- ISO 8601:2004 section 2.3.3 basic format
- ISO 8601 uses the word accuracy, not precision, in the relevant section, e.g: 2.3.7 representation with reduced accuracy.
- ISO 8601:2004(E). ISO. 2004-12-01. p. 8 Section 3.2.1.
- ISO 8601:2004(E). ISO. 2004-12-01. p. 6 with note.
- Nachum Dershowitz; Edward M. Reingold (2007-12-10). Calendrical Calculations. Cambridge University Press. p. 48-50. ISBN 978-0521702386.
- ISO 8601:2004(E). ISO. 2004-12-01. p. 3 Section 2.1.12.
- The Analysis Module of ESA’s Space Trajectory Analysis software Ana Margarida Teixeira Pinto Raposo Instituto Superior Técnico, Portugal, December 2010
- ISO 8601:2004(E). ISO. 2004-12-01. section 18.104.22.168 General.
- ISO 8601:2004(E). ISO. 2004-12-01. "3.5 Expansion ... By mutual agreement of the partners in information interchange, it is permitted to expand the component identifying the calendar year, which is otherwise limited to four digits. This enables reference to dates and times in calendar years outside the range supported by complete representations, i.e. before the start of the year  or after the end of the year ."
- ISO 8601:2004 sections 3.4.2, 22.214.171.124
- For example, see Annex B.1.1 of the standard.
- ISO 8601:2004 section 126.96.36.199 Representations with reduced accuracy
- "NIST TechBeat 2006-11-22". NIST. 2006-11-22. Retrieved 2012-05-06. "Decimals Score a Point on International Standards ... It soon may be possible to write international standards documents with decimal points in them."
- ISO 8601:2004(E), ISO, 2004-12-01, "188.8.131.52 ... the decimal fraction shall be divided from the integer part by the decimal sign specified in ISO 31-0, i.e. the comma [,] or full stop [.]. Of these, the comma is the preferred sign."
- RFC 3339 – Unknown local offset convention
- "ISO 8601:2004(E)". ISO. 2004-12-01. "4.3.2 NOTE: By mutual agreement of the partners in information interchange, the character [T] may be omitted in applications where there is no risk of confusing a date and time of day representation with others defined in this International Standard."
- ISO 8601:2004 section 184.108.40.206 Alternative format
- "Info on ISO 8601, the date and time representation standard". Cs.tut.fi. Retrieved 2012-08-29.
- "ISO 8601 - Getting with the Times (and Dates)". Hydrogold. 2012-01-01. Retrieved 2013-08-13.
- Note about Date and Time Formats to W3C from Reuters
- RFC 3339, section 5.6
- Nilsson, M. (2000-11-01). "ID3 tag version 2.4.0 - Main Structure". id3.org. pp. §4. Retrieved 2009-09-27.
- ISO's catalog entry for ISO 8601:2004
- Use international date format (ISO) – Quality Web Tips The World Wide Web Consortium (W3C)
- ISO 8601 at DMOZ
- ISO 8601 summary by Markus Kuhn
- Summary of 8601 by ISO at the Wayback Machine (archived 14 June 2011)
- Technical Committee ISO/TC 154, Processes, data elements and documents in commerce, industry and administration.
- The Mathematics of the ISO 8601 Calendar
- W3C Specification about UTC Date and Time and IETF Internet standard RFC 3339, based on ISO 8601