ANSI
DRAFT – October 20, 1997-  NCITS 310-199x
[Doc No: X3L8/97-008gV10]
Revision, redesignation, and consolidation of
ANSI X3.43-1991 and ANSI X3.51-1994

American National Standard
for Information Technology—
Representation of Time for Information Interchange

Secretariat
Information Technology Industry Council (ITI)

Approved (to come)
American National Standards Institute, Inc.

Forward (This forward is not part of the American National Standard NCITS 310-199X)

This standard provides a single, unambiguous format for representing time in information interchange.   Time refers to an instant in the time continuum of a day.  The basic representation of time for information interchange specified in this standard is HHMMSS.ssss plus a time-differential from coordinated universal time, where ‘HH’ refers to the hour in the twenty-four hour time keeping system, ‘MM’ refers to the minute, ‘SS’ refers to the second, and ‘ssss’ refers to the decimal second (to whatever number of significant digits as may me necessary.  This standard will be of interest to information and data managers, data administrators, and others responsible for making data sharable.  It will also be of interest to manufacturers of repository and CASE tool products.

This standard addresses the need for a single standard for information interchange of time data.  It combines two  previous ANSI standards on time in order to reduce potential confusion possible with  two separate standards relating to time.   A single standard for representation of time that incorporates all relevant aspects of the representation of time is desirable in order to reduce potential confusion in interchange of time information.   Other standards for representing time in information interchanges contain various options for interchange of time; thus, potential confusion can arise as to the exact format being used for interchange of time.  This standard eliminates such potential confusion.

This standard was developed within the usual consensus process of ANSI standards committees.   Knowledgeable experts from the U.S. Naval Observatory, the National Institute for Standards and Technology, Lawrence Berkeley Laboratory, and other organizations participated in the development of this standard.  Issues were fully discussed, with pros and cons identified, and votes taken to resolve them.

Requests for interpretation, suggestions for improvement or addenda, or defect reports are welcome.  They should be sent to National Committee for Information Technology Standards (NCITS), ITI, 1250 Eye Street, NW>, Suite 200, Washington, DC 20005.

This standard was processed and approved for submittal to ANSI by NCITS.  Committee approval of this standard does not necessarily imply that all committee members voted for its approval.  At the time it approved this standard, NCITS had the following members:

Ms. Karen Higginbottom, NCITS Chairman (Acting)
Ms. Karen Higginbottom, NCITS Vice Chairman
Ms. Monica Vago, NCITS Secretary

AMP Incorporated
Mr. John Hill (P)
Mr. Charles Brill (A)

Apple Computer Inc.
Mr. David K. Michael (P)
Mr. Jerry Kellenbenz (A)

AT&T
Mr. Thomas F. Frost (P)
Mr. Paul Bartoli (A)

Bull HN Information Systems Inc
Mr. Patrick L. Harris (P)

Compaq Computers
Mr. Stephen Heil (P)
Mr. Steve Park (A)

Digital Equipment Corporation
Mr. Scott K. Jameson (P)
Mr. Richard Hovey (A)

Eastman Kodak Company
Mr. Michael Nier (P)

Hewlett-Packard
Ms. Karen Higginbottom (P)
Mr. Donald C. Loughry (A)

Hitachi America Ltd.
Mr. John Neumann (P)
Mr. Kei Yamashita (A)

Hughes Aircraft Company
Mr. Harold Zebrack (P)

IBM Corporation
Mr. Ronald Sellitti (P)
Mr. Joel Urman (A)

Institute for Certification of Computer Professionals (ICCP)
Mr. Kenneth Zemrowski (P)

Imation
Mr. Philip E. Friedlund (P)

Lucent Technologies
Mr. Herbert Bertine (P)
Mr. Tom Rutt (A)

National Communications Systems
Mr. Dennis Bodson (P)
Mr. William Olden (A)
Mr. Frank McClelland (A)

National Institute of Standards & Technology
Mr. Michael D. Hogan (P)

Panasonic Technologies
Mr. Y. Machida (P)

Share Inc.
Mr. Gary Ainsworth (P)
Mr. David Thewlis (A)

Sony
Mr. Masataka Ogawa (P)
Mr. Michael Deese (A)

Storage Technology Corporation
Mr. Joseph S. Zajaczkowski (P)

Sun Microsystems Inc.
Mr. Gary S. Robinson (P)

Sybase Inc.
Mr. Donald R. Deutsch (P)

Texas Instruments Inc.
Mr. Clyde Camp (P)
Mr. Fritz Whittington (A)

Unisys Corporation
Mr. Arnold Winkler (P)
Mr. Stephen P. Oksala (A)

U.S. Department of Defense
Mr. Jerry L. Smith (P)
Mr. C.J. Pasquariello (A)

U.S. Department of Energy
Ms. Carol S. Blackston (P)
Mr. Bruce R. White (A)

Xerox Corporation
Mr. John Flannery (P)
Mr. Roy Pierce (A)

The ANSI NCITS L8 Technical Committee on Data Representation, which developed this standard had the following members:

Phong X. Ngo, Chair, Science Applications International Corporation (P)
Bruce E. Bargmeyer, Vice Chair, U.S. Environmental Protection Agency (P)
Daniel W. Gillman, Special Interest Group Chair, U.S. Bureau of the Census (P)
Judith Newton, Special Interest Group Vice Chair, National Institute of Standards and Technology (P)

Battelle Memorial Institute……………………………….……………..	Douglas D. Mann (P)
Bell Communications Research…………………………………………	Paul M. Hawes (P)
								Paul R. Levine (A)
Department of the Army…………………………………………………	Bruce Haberkamp (P)	
Lawrence Berkeley National Laboratory………………………………..	Frank Olken (P)
								John McCarthy (A)
Meta4 Incorporated……………………………………………………..	Revere Perkins (P)
								William P. Doyle (A)
Mitertek Incorporated…………………………………………..………	Andrew M. Schoka (P)
National Institute of Standards and Technology………………………..	Bruce K. Rosen (A)
Paladin Integration Engineering……………………………………..….	Burton G. Parker (P)
San Diego Supercomputer Center……………………………………..	Arcot Rajasekar (P)
Science Applications International Corporation……………………..…	Joseph Leahy (A)
self……………………………………………………………..……….	Henry Heffernan
self………………………………………………………………………	William H. Kenworthey, Jr. (O)
Space Applications Corporation……………………….……………….	Peter D. Noerdlinger (O)
U.S. Bureau of the Census………………………………………….…..	Martin V. Appel (A)
								William P. LaPlant (O)
U.S. Environmental Protection Agency……………………………..…..	Larry Fitzwater (A)
U.S. Geological Survey…………………………………………….……	Roger L. Payne (P)
							           	Louis A. Yost IV (A)
U.S. Naval Observatory…………………………………..……………..	Brent A. Archinal (O)
U.S. Veterans Administration……………………………………….…..	Philip H. Diamond (P)
University of South Carolina, Dept of Electronic & Computer Eng…….	Michael N. Huhns (P)
								Larry M. Stephens (A)

1    Scope, purpose, and application

1.1  Scope  
The scope of this American National Standard is limited to the representation of time for interchange among data systems; it does not describe how time is determined.  This standard was not designed for (nor does it preclude) usage by humans as input to, or output from, data systems. 

1.2  Purpose  
The purpose of this American National Standard is to provide a single, unambiguous standard means of representing time for interchange of time information among data systems.  

1.3  Application  
The representation of time specified in this standard is compatible with other national and international standards.  The representation specified by this American National Standard is in the basic format of “differences between civil time and Coordinated Universal Time” (as described in ISO 8601:1988), with an extension to represent designated civil time, which is defined to have the same numerical representation across all time zones.

1.4  Cancellations
This standard cancels and replaces American National Standard for Information Systems—Representations of Local Time of Day for Information Interchange (ANSI X3.43-1991) and American National Standard for Information Systems—Representations of Universal Time, Local Time Differentials, and United States Time Zone References for Information Interchange (ANSI X3.51-1994).  

2   Definitions
For this American National Standard, the following definitions apply:

2.1  time:  “In English ‘time’ is used to specify an instant (time of day)¼” [ITU-R Recommendations, 1994 TF Series Volume, Time Signals and Frequency Standards Emissions (Recommendation 686, Glossary)]. A particular point in the stream of time of at a particular place (which may or may not be specified in terms of a particular date):  a specific hour, or minute, or second, or fraction of a second in a day at a specific place.

2.2  second:  “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 cesium atom-133.” (XIIIe  Conference Generale des Poids et Mesures, 1967).  Ordinarily, a second is one-sixtieth of a minute.

2.3  leap second:  A second of time intentionally inserted in or deleted from the UTC time stream to keep it approximately compatible with the rotation of the Earth .  An inserted second is called a positive leap second and an omitted second is called a negative leap second.  For the purposes of this standard, a positive leap second is labeled “60” and a negative leap second is labeled “58” (there being no second with the label “59” in a minute with a negative leap second).

2.4  minute:  Sixty seconds, each labeled consecutively in the 24-hour timekeeping system from 0 through 59 beginning with the start of the first second after the beginning of the minute.  Ordinarily, a minute is one-sixtieth of an hour.  
Note: a minute which contains a leap second will have 61 or 59 seconds, depending upon whether the leap second is positive or negative, respectively.

2.5  hour:  Sixty minutes, each labeled consecutively in the 24-hour timekeeping system from 0 through 23 beginning with the start of the first minute after the beginning of the hour.  Ordinarily, an hour is one twenty-fourth of a day.  
Note: an hour which contains a leap second will have will have one extra or one less second, depending upon whether the leap second is positive or negative, respectively.

2.6  date:  A particular day within a Gregorian calendar month, consisting of 24 hours in the 24-hour timekeeping system.  Ordinarily, a date is specified by a year, a month, and the sequential day within a month  (See ANSI X3.30).  
Note: A day that contains a leap second will have one extra or one less second, depending upon whether the leap second is positive or negative, respectively.

2.7  designated civil time:  A time whose representation is independent of all time differentials from UTC.   Example:  9 o’clock in the morning civil time in any time zone, adjusted for Daylight Saving Time, if appropriate.

2.8  civil time:  The time defined in a region by the civil authorities there, adjusted for Daylight Saving Time, if appropriate.

2.9  Daylight Saving Time:  An instant in civil time represented with an adjustment specified by the Uniform Time Act of 1965, as amended:  “During the period commencing at 2 a.m. on the first Sunday in April of each year and ending at 2 a.m. on the last Sunday in October, the standard time is advanced one hour except in those states that have by law exempted themselves from observance of advanced time.”  
Note: ‘standard time’ as used in this quotation is taken to refer to civil time for the purposes of this standard.

2.10 coordinated universal time (UTC):  “The official coordinate time scale for the Earth defined on the rotating geoid.” [Rec.ITU-R TF.1010, Relativistic Effects in a Coordinate Time System in the Vicinity of the Earth].    “The time scale, maintained by the Bureau International des Poids et Mesures (BIPM), and the International Earth Rotation Service (IERS), which forms the basis of a coordinated dissemination of standard frequencies and time signals” [Rec 686, Glossary].

2.11  Time-differential-from-UTC:  The difference in hours and minutes between Civil time and UTC.

3   Specifications

3.1  Time for information interchange
Time is to be represented as civil non-daylight saving time plus a time-differential-from-UTC in the following “24 hour clock” format:  HHMMSS.ssss(+, - , or *) hhmm, 

  where 
	HH represents hours, MM represents minutes, SS represents seconds, ssss represents decimal fractions of a second with an unlimited number of significant digits, 
	
 and
	“+” or “-”  represents positive or negative time differentials (respectively) from UTC, 
	
 or
	 “*” represents all time differentials from UTC worldwide,
	
 with
	 hhmm representing hours and minutes time-differential-from-UTC. 
	
Note:  The parenthesis symbols are not part of the representational format, but are used to indicate that representations of time will  use one of the three symbols.
	
Example: 39 minutes and 25.6 seconds past 7 o’clock EST civil time in the afternoon of 31 December in Philadelphia, Pennsylvania is a point in non-daylight saving time represented for interchange purposes as 193925.6-0500.

Example:  Designated civil time for 7 o’clock in the afternoon civil time in every time zone in the world, with one-decimal-second significant digit, is represented as 190000.0*  
Note: time-differential-from-UTC is not present in the representation of a designated civil time

3.2  Sequencing of time components
The sequencing of time components shall be from high order to low order (left to right):  hour of the day (HH), minute of the hour (MM), second of the minute (SS) [with decimal seconds as appropriate: ssss], followed by the time-differential-from-UTC.   Leading zeroes in each time component are required.  Truncation of higher order time components (from left to right) is not permitted.

3.3  Degree of specificity
The degree of specificity associated with time in specific information interchanges may require truncation of lower order time components (from right to left).  Thus, decimal fractions of a second may be dropped, seconds and decimal fractions of a second may be dropped, or minutes and seconds and decimal fractions of a second may be dropped.  
Note: the time-differential-from-UTC may NOT be dropped, except in representations of  designated civil time as defined below.

3.4  Use of separators
No separators are allowed in time representations for information interchange, other than those specified in this standard.  

3.5  Representation of seconds  
The beginning of a second within a minute of an hour of a day shall be represented by a two-digit decimal number ranging from 00 through 60.  When a decimal fraction of a second is specified (ssss), it shall be separated from the second representation by a decimal point (period) and expressed numerically to the precision (number of significant digits) desired. 
Note: The allowable two-digit decimal numbers for seconds ordinarily range from 00 through 59.  The two-digit decimal number 60 is reserved for representing a positive leap second.   The allowed two-digit decimal numbers range from 00 to only 58 in the last minute of a day  having a negative leap second.
  
3.6  Representation of minutes  
The beginning of a minute within an hour of the day shall be represented by a two-digit decimal number ranging from 00 through 59.

3.7  Representation of Hours  
The beginning of an hour of day shall be represented using the 24 hour timekeeping  system by recording a two-digit decimal number ranging from 00 through 23.

3.8  Representation of time-differential-from-UTC  
The hours (hh) and minutes (mm) offset from UTC, expressed as a four digit number (hhmm),  are mandatory for all representations of time for information interchange except that for designated civil time specified below.  A plus (+) or minus  (-) precedes the four digit number (which may be zeros), indicating the hours and minutes the civil time is ahead of or behind UTC, respectively.

3.9  Representation of  UTC  
A specific UTC time shall be represented as HHMMSS.ssss+0000.  Alternatively, the ‘+0000’ notation may be replaced by a ‘Z’ notation, as a capital letter only.

3.10  Representation of designated civil time
A specified civil time independent of time zone shall be represented as HHMMSS.ssss*

3.11  Representation of midnight and noon
Midnight shall be represented in hours, minutes, and seconds as 000000 (beginning of the day).  Noon shall be represented in hours, minutes, and seconds as 120000.

4.  Combinations of Date and Time Representation

This standard may be used in combination with ANSI X3.30-1997.  High-order to low-order sequence shall be maintained, that is:  year, month, day, hour, minute, second (and then fraction thereof).  Separators, other than those specified  in this standard, are not permitted.   

Example:  39 minutes and 25.6 seconds past 7 o’clock EDT civil time in the afternoon of July 4th, 1997 in Philadelphia, Pennsylvania is a point in Daylight Saving Time that must be converted to non-daylight-saving-time (6 o’clock) in its representation for interchange purposes:  19970704183925.6-0500.

Annex A
(normative)

Relationships between representations of time for information interchange,
 Daylight Saving Time, and  U.S. Time Zone code sets 

A.1  Interchange versus alternate representations of time.  

A.1.1  Although this standard specifies a single, unambiguous representation of time for information interchange, data systems often require output or input of more user-friendly representations of time.  These representations include Daylight Saving Time and time zone reference formats for representation of time.  This annex specifies how conversions are made between representations of time for information interchange and the more user-oriented Daylight Saving Time and the U.S. Time Zone code sets.  

A.1.2  Representations of time in Time Zone or Daylight Saving Time formats shall NOT be used for information interchange among data systems.

A.2  U.S. Time Zone codes

A.2.1  Representation of civil time zones in the United States are shown in table A.1.  This table gives the name of each time zone in the United States and its territories, together with its representational code, its time zone designator, and its time-differential-from-UTC (TDU)

Table A-1- Time zone references for U.S. civil time zones
Time zone reference code	Time zone designator	Standard time zone name	TDU	
AST	Q	Atlantic Standard Time	-0400	
EST	R	Eastern Standard Time	-0500	
CST	S	Central Standard Time	-0600	
MST	T	Mountain Standard Time	-0700	
PST	U	Pacific Standard Time	-0800	
KST	V	Alaska Standard Time	-0900	
HST	W	Hawaii-Aleutian Standard Time	-1000	
SST	X	Samoa Standard Time	-1100	

A2.2  Conversion of representations of time for information interchange to time representations that include standard time zone reference codes is made by identifying the time zone reference code associated with the time-differential-from-UTC component of the representation of time for information interchange.  Note:  standard time zones are taken to refer to civil time zones for the purposes of this standard.

A time zone reference code is represented by the uppercase three-letter code given in table A.1.  It is separated from the lower-order time component of a civil time expression by a space.

Example:  A time zone representational form for 193925.6-0500 would be 193925.6 EST.

Note: Other manipulations of representations of time for information interchange may be required within a data system to provide more human-readable output formats.   Such conversions may include conversion to the 12 hour timekeeping system and the introduction of separators.  
Example:  193925.6-0500 may be converted for data system output as 7:39:25.6 p.m. EST.

A.2.3  Conversion of time zone reference code representations of time to representations of time for information interchange is accomplished in the reverse of the process described above.  That is, the time-differential-from-UTC associated with the specific time zone reference code is identified from table A-1 and used to construct the equivalent representation of time for information interchange.

A.3  Daylight Saving Time

Another conversion of representations of time for information interchange may require adjustment by data systems to output time in Daylight Saving Time format.  

A.3.1  In accordance with the Uniform Time Act of 1965, as amended, “During the period commencing at 2 a.m. on the first Sunday in April of each year and ending at 2 a.m. on the last Sunday in October, the civil time is advanced one hour except in those states that have by law exempted themselves from observance of advanced time.”  This advanced time is commonly referred to as “Daylight” or “Daylight Saving” time.   Table A.2 gives the name of each daylight time zone in the United States and its territories, together with its representational code, its time zone designator, and its adjusted time-differential-from-UTC (TDU).

Table A-2 - Time zone references for U.S. standard time zones
Time zone reference code	Time zone designator	Daylight time zone name	Adjusted TDU	
ADT	Q	Atlantic Daylight Time	-0300	
EDT	R	Eastern Daylight Time	-0400	
CDT	S	Central Daylight Time	-0500	
MDT	T	Mountain Daylight Time	-0600	
PDT	U	Pacific Daylight Time	-0700	
KDT	V	Alaska Daylight Time	-0800	
HDT	W	Hawaii-Aleutian Daylight Time	-0900	
SDT	X	Samoa Daylight Time	-1000	

A.3.2  Conversion of representations of time for information interchange to time representations that include daylight time zone reference codes is made by adding one hour to the time-differential-from-UTC component of representation of time for information interchange and identifying the daylight time zone reference code associated with this adjusted time-differential-from-UTC.  This daylight time zone reference code and the new hour component are used to construct the representation of time in daylight time zone reference code format.  Note:  standard time zones are taken to refer to civil time zones for the purposes of this standard.

A daylight time zone reference code is represented by the uppercase three-letter code given in table A.2.  It is separated from the lower-order time component of a civil time expression by a space.

Example:  A Daylight Saving Time, time zone representational form for 193925.6-0500 would be 203925.6 EDT.
Note:  Other manipulations of representations of time for information interchange may be required within a data system to provide more human-readable output formats.   Such conversions may include conversion to the 12 hour timekeeping system and the introduction of separators.  
Example:  193925.6-0500 may be converted for data systems output as 8:39:25.6 p.m. EDT.

A3.3  Conversion of daylight time zone reference code representations of time to representations of time for information interchange is accomplished in the reverse of the process described above.  That is, the adjusted time-differential-from-UTC associated with the specific daylight time zone reference code is identified from table A.2.  One hour is subtracted from this adjusted time-differential-from-UTC to produce the time-differential-from-UTC needed for the representation of time for information interchange. This new time-differential-from-UTC component is used to construct the equivalent representation of time for information interchange.

Annex B
(informative)

Explanation of Designated Civil Time

B.1  Designated civil time  

B.1.1  This standard specifies a designated civil time, which is a time whose representation is independent of all time differentials from UTC.  This annex explains this concept.

B.1.2  The concept of a designated civil time is explained in terms of an example of its use.   The example is illustrated by examining the nature of a policy of a delivery service, the policy being that it will guarantee overnight delivery by a specific time, say noon, anywhere in the world for items submitted for delivery by noon of the day the item is submitted.    

B.2  Implications of ‘overnight’ delivery   

B.2.1  The promise of a delivery service to deliver anywhere in the world by noon of the next day, means that the service will deliver to the addressee by noon civl time of the next day anywhere in the world, provided the item for delivery is received from the sender by noon.    The examples below assume that no Daylight Saving Time is in effect.

Example 1.  Assume a sender delivers an item to the delivery service by noon civil time on 2 January 1997 in Auckland, New Zealand for overnight delivery to New York, New York, USA.   The sender of the item would then expect that the receiver would have the item in hand by noon civil time in New York on 3 January 1997.  The delivery service would have 16 hours to deliver the item , since noon civil time on 1 January in Auckland is 8 p.m. civil time on 1 January in New York.    

Example 2.  Assume a sender delivers an item to the delivery service by noon civil time on 2 January 1997 in Auckland, New Zealand for overnight delivery to Hong Kong.   The sender of the item would still expect that the receiver would have the item in hand by noon civil time in Hong Kong on 3 January 1997.  But now the delivery service would have 29 hours to deliver the item , since noon civil time on 2 January in Auckland is 7 a.m. civil time on 3 January in Hong Kong.

B.2.2   Designated civil time is the mechanism for specifying a particular civil time in any time zone, in this case 120000, in hours, minutes, and seconds.  The representation of this designated civil time is 120000*.  Using ANSI X3.30 in conjunction with this designated civil time, a representation of the delivery service provider’s policy on delivery for items received by noon civil time on 2 January 1997 would take the form:  For items received by 19970201120000*, delivery will be accomplished by 19970103120000*.  Or, in plain English:  For items received by 12 o’clock noon civil time anywhere in the world on 2 January 1997, delivery will be accomplished to any other place in the world by 12 o’clock civil time on 3 January 1997.

Annex C
(informative)

Sources of Time Information

C.1  Sources of UTC timing information  

C.1.1  U. S. Naval Observatory

	U. S. Naval Observatory
	Time Service Department
	3450 Massachusetts Avenue, NW
	Washington DC 20392-5420

C.1.2  National Institute of Standards and Technology (NIST)

	National Institute of Standards and Technology 
	Time and Frequency Division
	325 Broadway
	Boulder CO 80303

C.1.3  Bureau International des Poids et Mesures (BIPM)

	Bureau International des Poids et Mesures (BIPM)
	Time Section
	Pavillon de Breteuil
	F-92312 Sevres Cedex
	France

C.2  Sources of U. S. time zone information  

C.2.1  U. S. Department of Transportation

	Time
	c/o Office of General Counsel
	U. S. Department of Transportation
	Washington DC 20590

C.3  Sources of  leap second information  

C.3.1  International Earth Rotation Service (IERS)

	International Earth Rotation Service (IERS Bulletin C)
	Central Bureau (IERS/CB)
	Observatoire de Paris
	61, avenue de l’Observatoire
	75014 Paris
	France
	
C.3. 2  U. S. National Earth Orientation Service (NEOS)

	U. S. National Earth Orientation Service
	c/o U. S. Naval Observatory
	3450 Massachusetts Avenue, NW
	Washington DC 20392-5420

C.4  Sources of information for dating of events in the vicinity of a leap second

C.4.1  ITU-R Recommendations, 1994 TF Series Volume, Time Signals and Frequency Standards Emissions

	Annex III of Recommendation 460-4, “Standard-Frequency and Time-Signal Emissions” of
	ITU-R Recommendations, 1994 TF Series Volume, Time Signals and Frequency Standards Emissions, page 17.

International Telecommunications Union
Place des Nations
1211 Geneva 20
Switzerland

Annex D
(Informative)

Bibliography

XIIIe  Conference Generale des Poids et Mesures, 1967

ANSI X3.30-1971 (R1997),  Representation of Date for Information Interchange

ISO 8601:1988, Data elements and interchange formats -- Information interchange -- Representation of dates and times.

ITU-R Recommendations, 1994 TF Series Volume, Time Signals and Frequency Standards Emissions (Recommendation 686, Glossary)

Rec.ITU-R TF.1010, Relativistic Effects in a Coordinate Time System in the Vicinity of the Earth