Date and Time on the Internet: Timestamps

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.¶

Copyright Notice

Abstract

This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar.¶

1. Introduction

Date and time formats cause a lot of confusion and interoperability problems on the Internet. This document addresses many of the problems encountered and makes recommendations to improve consistency and interoperability when representing and using date and time in Internet protocols.¶

This document includes an Internet profile of the ISO 8601 [ISO8601] standard for representation of dates and times using the Gregorian calendar.¶

There are many ways in which date and time values might appear in Internet protocols: this document focuses on just one common usage, viz. timestamps for Internet protocol events. This limited consideration has the following consequences: ¶

All dates and times are assumed to be in the "current era", somewhere between 0000AD and 9999AD.
All times expressed have a stated relationship (offset) to Coordinated Universal Time (UTC). (This is distinct from some usage in scheduling applications where a local time and location may be known, but the actual relationship to UTC may be dependent on the unknown or unknowable actions of politicians or administrators. The UTC time corresponding to 17:00 on 23rd March 2005 in New York may depend on administrative decisions about daylight savings time. This specification steers well clear of such considerations.)
Timestamps can express times that occurred before the introduction of UTC. Such timestamps are expressed relative to universal time, using the best available practice at the stated time.
Date and time expressions indicate an instant in time. Description of time periods, or intervals, is not covered here.

2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].¶

UTC: Coordinated Universal Time as maintained by the Bureau International des Poids et Mesures (BIPM).
second: A basic unit of measurement of time in the International System of Units. It is defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.
minute: A period of time of 60 seconds. However, see also the restrictions in Section 5.7 and Appendix D for how leap seconds are denoted within minutes.
hour: A period of time of 60 minutes.
day: A period of time of 24 hours.
leap year: In the Gregorian calendar, a year which has 366 days. A leap year is a year whose number is divisible by four an integral number of times, except that if it is a centennial year (i.e. divisible by one hundred) it shall also be divisible by four hundred an integral number of times.
ABNF: Augmented Backus-Naur Form, a format used to represent permissible strings in a protocol or language, as defined in [ABNF].
Email Date/Time Format: The date/time format used by Internet Mail as defined by RFC 2822 [IMAIL-UPDATE].
Internet Date/Time Format: The date format defined in section 5 of this document.
Timestamp: This term is used in this document to refer to an unambiguous representation of some instant in time.
Z: A suffix which, when applied to a time, denotes a UTC offset of 00:00; often spoken "Zulu" from the ICAO phonetic alphabet representation of the letter "Z".

For more information about time scales, see Appendix E of [NTP], Section 3 of [ISO8601], and the appropriate ITU documents [ITU-R-TF].¶

3. Two Digit Years

The following requirements are to address the problems of ambiguity of 2-digit years: ¶

Internet Protocols MUST generate four digit years in dates.
The use of 2-digit years is deprecated. If a 2-digit year is received, it should be accepted ONLY if an incorrect interpretation will not cause a protocol or processing failure (e.g. if used only for logging or tracing purposes).
It is possible that a program using two digit years will represent years after 1999 as three digits. This occurs if the program simply subtracts 1900 from the year and doesn't check the number of digits. Programs wishing to robustly deal with dates generated by such broken software may add 1900 to three digit years.
It is possible that a program using two digit years will represent years after 1999 as ":0", ":1", ... ":9", ";0", ... This occurs if the program simply subtracts 1900 from the year and adds the decade to the US-ASCII character zero. Programs wishing to robustly deal with dates generated by such broken software should detect non-numeric decades and interpret appropriately.

The problems with two digit years amply demonstrate why all dates and times used in Internet protocols MUST be fully qualified.¶

4. Local Time

4.1. Coordinated Universal Time (UTC)

Because the daylight saving rules for local time zones are so convoluted and can change based on local law at unpredictable times, true interoperability is best achieved by using Coordinated Universal Time (UTC). This specification does not cater to local time zone rules.¶

4.2. Local Offsets

The offset between local time and UTC is often useful information. For example, in electronic mail (RFC2822, [IMAIL-UPDATE]) the local offset provides a useful heuristic to determine the probability of a prompt response. Attempts to label local offsets with alphabetic strings have resulted in poor interoperability in the past [IMAIL], [HOST-REQ]. As a result, RFC2822 [IMAIL-UPDATE] has made numeric offsets mandatory.¶

Numeric offsets are calculated as "local time minus UTC". So the equivalent time in UTC can be determined by subtracting the offset from the local time. For example, 18:50:00-04:00 is the same time as 22:50:00Z. (This example shows negative offsets handled by adding the absolute value of the offset.) ¶

NOTE: Following ISO 8601, numeric offsets represent only time zones that differ from UTC by an integral number of minutes. However, many historical time zones differ from UTC by a non- integral number of minutes. To represent such historical time stamps exactly, applications must convert them to a representable time zone.

4.3. Unknown Local Offset Convention

If the time in UTC is known, but the offset to local time is unknown, this can be represented with an offset of "-00:00". This differs semantically from an offset of "Z" or "+00:00", which imply that UTC is the preferred reference point for the specified time. RFC2822 [IMAIL-UPDATE] describes a similar convention for email.¶

4.4. Unqualified Local Time

A number of devices currently connected to the Internet run their internal clocks in local time and are unaware of UTC. While the Internet does have a tradition of accepting reality when creating specifications, this should not be done at the expense of interoperability. Since interpretation of an unqualified local time zone will fail in approximately 23/24 of the globe, the interoperability problems of unqualified local time are deemed unacceptable for the Internet. Systems that are configured with a local time, are unaware of the corresponding UTC offset, and depend on time synchronization with other Internet systems, MUST use a mechanism that ensures correct synchronization with UTC. Some suitable mechanisms are: ¶

Use Network Time Protocol [NTP] to obtain the time in UTC.
Use another host in the same local time zone as a gateway to the Internet. This host MUST correct unqualified local times that are transmitted to other hosts.
Prompt the user for the local time zone and daylight saving rule settings.

5. Date and Time format

This section discusses desirable qualities of date and time formats and defines a profile of ISO 8601 for use in Internet protocols.¶

5.1. Ordering

If date and time components are ordered from least precise to most precise, then a useful property is achieved. Assuming that the time zones of the dates and times are the same (e.g., all in UTC), expressed using the same string (e.g., all "Z" or all "+00:00"), and all times have the same number of fractional second digits, then the date and time strings may be sorted as strings (e.g., using the strcmp() function in C) and a time-ordered sequence will result. The presence of optional punctuation would violate this characteristic.¶

5.2. Human Readability

Human readability has proved to be a valuable feature of Internet protocols. Human readable protocols greatly reduce the costs of debugging since telnet often suffices as a test client and network analyzers need not be modified with knowledge of the protocol. On the other hand, human readability sometimes results in interoperability problems. For example, the date format "10/11/1996" is completely unsuitable for global interchange because it is interpreted differently in different countries. In addition, the date format in [IMAIL] has resulted in interoperability problems when people assumed any text string was permitted and translated the three letter abbreviations to other languages or substituted date formats which were easier to generate (e.g. the format used by the C function ctime). For this reason, a balance must be struck between human readability and interoperability.¶

Because no date and time format is readable according to the conventions of all countries, Internet clients SHOULD be prepared to transform dates into a display format suitable for the locality. This may include translating UTC to local time.¶

5.3. Rarely Used Options

A format which includes rarely used options is likely to cause interoperability problems. This is because rarely used options are less likely to be used in alpha or beta testing, so bugs in parsing are less likely to be discovered. Rarely used options should be made mandatory or omitted for the sake of interoperability whenever possible.¶

The format defined below includes only one rarely used option: fractions of a second. It is expected that this will be used only by applications which require strict ordering of date/time stamps or which have an unusual precision requirement.¶

5.4. Redundant Information

If a date/time format includes redundant information, that introduces the possibility that the redundant information will not correlate. For example, including the day of the week in a date/time format introduces the possibility that the day of week is incorrect but the date is correct, or vice versa. Since it is not difficult to compute the day of week from a date (see Appendix B), the day of week should not be included in a date/time format.¶

5.5. Simplicity

The complete set of date and time formats specified in ISO 8601 [ISO8601] is quite complex in an attempt to provide multiple representations and partial representations. Appendix A contains an attempt to translate the complete syntax of ISO 8601 into ABNF. Internet protocols have somewhat different requirements and simplicity has proved to be an important characteristic. In addition, Internet protocols usually need complete specification of data in order to achieve true interoperability. Therefore, the complete grammar for ISO 8601 is deemed too complex for most Internet protocols.¶

The following section defines a profile of ISO 8601 for use on the Internet. It is a conformant subset of the ISO 8601 extended format. Simplicity is achieved by making most fields and punctuation mandatory.¶

5.6. Internet Date/Time Format

The following profile of ISO 8601 [ISO8601] dates SHOULD be used in new protocols on the Internet. This is specified using the syntax description notation defined in [ABNF].¶

   date-fullyear   = 4DIGIT
   date-month      = 2DIGIT  ; 01-12
   date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                             ; month/year
   time-hour       = 2DIGIT  ; 00-23
   time-minute     = 2DIGIT  ; 00-59
   time-second     = 2DIGIT  ; 00-58, 00-59, 00-60 based on leap second
                             ; rules
   time-secfrac    = "." 1*DIGIT
   time-numoffset  = ("+" / "-") time-hour ":" time-minute
   time-offset     = "Z" / time-numoffset

   partial-time    = time-hour ":" time-minute ":" time-second
                     [time-secfrac]
   full-date       = date-fullyear "-" date-month "-" date-mday
   full-time       = partial-time time-offset

   date-time       = full-date "T" full-time

NOTE: Per [ABNF] and ISO8601, the "T" and "Z" characters in this syntax may alternatively be lower case "t" or "z" respectively.
This date/time format may be used in some environments or contexts that distinguish between the upper- and lower-case letters 'A'-'Z' and 'a'-'z' (e.g. XML). Specifications that use this format in such environments MAY further limit the date/time syntax so that the letters 'T' and 'Z' used in the date/time syntax must always be upper case. Applications that generate this format SHOULD use upper case letters.
NOTE: ISO 8601 defines date and time separated by "T". Applications using this syntax may choose, for the sake of readability, to specify a full-date and full-time separated by (say) a space character.

5.7. Restrictions

The grammar element date-mday represents the day number within the current month. The maximum value varies based on the month and year as follows:¶

Month Number	Month/Year	Maximum value of date-mday
01	January	31
02	February, normal	28
02	February, leap year	29
03	March	31
04	April	30
05	May	31
06	June	30
07	July	31
08	August	31
09	September	30
10	October	31
11	November	30
12	December	31

Appendix C contains sample C code to determine if a year is a leap year.¶

The grammar element time-second may have the value "60" at the end of months in which a leap second occurs -- to date: June (XXXX-06-30T23:59:60Z) or December (XXXX-12-31T23:59:60Z); see Appendix D for a table of leap seconds. It is also possible for a leap second to be subtracted, at which times the maximum value of time-second is "58". At all other times the maximum value of time-second is "59". Further, in time zones other than "Z", the leap second point is shifted by the zone offset (so it happens at the same instant around the globe).¶

Leap seconds cannot be predicted far into the future. The International Earth Rotation Service publishes bulletins [IERS] that announce leap seconds with a few weeks' warning. Applications should not generate timestamps involving inserted leap seconds until after the leap seconds are announced.¶

Although ISO 8601 permits the hour to be "24", this profile of ISO 8601 only allows values between "00" and "23" for the hour in order to reduce confusion.¶

5.8. Examples

Here are some examples of Internet date/time format.¶

      1985-04-12T23:20:50.52Z

This represents 20 minutes and 50.52 seconds after the 23rd hour of April 12th, 1985 in UTC.

      1996-12-19T16:39:57-08:00

This represents 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC (Pacific Standard Time). Note that this is equivalent to 1996-12-20T00:39:57Z in UTC.

      1990-12-31T23:59:60Z

This represents the leap second inserted at the end of 1990.

      1990-12-31T15:59:60-08:00

This represents the same leap second in Pacific Standard Time, 8 hours behind UTC.

      1937-01-01T12:00:27.87+00:20

This represents the same instant of time as noon, January 1, 1937, Netherlands time. Standard time in the Netherlands was exactly 19 minutes and 32.13 seconds ahead of UTC by law from 1909-05-01 through 1937-06-30. This time zone cannot be represented exactly using the HH:MM format, and this timestamp uses the closest representable UTC offset.

6. References

[REF: In the published official version of the RFC, this section contains the references (here: Section 8).]¶

7. Security Considerations

Since the local time zone of a site may be useful for determining a time when systems are less likely to be monitored and might be more susceptible to a security probe, some sites may wish to emit times in UTC only. Others might consider this to be loss of useful functionality at the hands of paranoia.¶

8. References

[ZELLER]: Zeller, C., “Kalender-Formeln”, Acta Mathematica Vol. 9, Nov 1886.
[IMAIL]: Crocker, D., “Standard for the format of ARPA Internet text messages”, STD 11, RFC 822, August 1982.
[IMAIL-UPDATE]: Resnick, P., “Internet Message Format”, RFC 2822, April 2001.
[ABNF]: Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, RFC 2234, November 1997.
[ISO8601]: International Organization for Standardization, “Data elements and interchange formats -- Information interchange -- Representation of dates and times”, ISO 8601:1988(E), June 1988.
[ISO8601-2000]: International Organization for Standardization, “Data elements and interchange formats -- Information interchange -- Representation of dates and times”, ISO 8601:2000, December 2000.
[HOST-REQ]: Braden, R., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, October 1989.
[IERS]: “International Earth Rotation Service Bulletins”, <http://hpiers.obspm.fr/eop-pc/products/bulletins.html>.
[NTP]: Mills, D., “Network Time Protocol (Version 3) Specification, Implementation”, RFC 1305, March 1992.
[ITU-R-TF]: “International Telecommunication Union Recommendations for Time Signals and Frequency Standards Emissions”, <http://www.itu.ch/publications/itu-r/iturtf.htm>.
[RFC2119]: Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

Appendix A. ISO 8601 Collected ABNF

This information is based on the 1988 version of ISO 8601. There may be some changes in the 2000 revision.¶

ISO 8601 does not specify a formal grammar for the date and time formats it defines. The following is an attempt to create a formal grammar from ISO 8601. This is informational only and may contain errors. ISO 8601 remains the authoritative reference.¶

Note that due to ambiguities in ISO 8601, some interpretations had to be made. First, ISO 8601 is not clear if mixtures of basic and extended format are permissible. This grammar permits mixtures. ISO 8601 is not clear on whether an hour of 24 is permissible only if minutes and seconds are 0. This assumes that an hour of 24 is permissible in any context. Restrictions on date-mday in section 5.7 apply. ISO 8601 states that the "T" may be omitted under some circumstances. This grammar requires the "T" to avoid ambiguity. ISO 8601 also requires (in section 5.3.1.3) that a decimal fraction be proceeded by a "0" if less than unity. Annex B.2 of ISO 8601 gives examples where the decimal fractions are not preceded by a "0". This grammar assumes section 5.3.1.3 is correct and that Annex B.2 is in error.¶

   date-century    = 2DIGIT  ; 00-99
   date-decade     =  DIGIT  ; 0-9
   date-subdecade  =  DIGIT  ; 0-9
   date-year       = date-decade date-subdecade
   date-fullyear   = date-century date-year
   date-month      = 2DIGIT  ; 01-12
   date-wday       =  DIGIT  ; 1-7  ; 1 is Monday, 7 is Sunday
   date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                             ; month/year
   date-yday       = 3DIGIT  ; 001-365, 001-366 based on year
   date-week       = 2DIGIT  ; 01-52, 01-53 based on year

   datepart-fullyear = [date-century] date-year ["-"]
   datepart-ptyear   = "-" [date-subdecade ["-"]]
   datepart-wkyear   = datepart-ptyear / datepart-fullyear

   dateopt-century   = "-" / date-century
   dateopt-fullyear  = "-" / datepart-fullyear
   dateopt-year      = "-" / (date-year ["-"])
   dateopt-month     = "-" / (date-month ["-"])
   dateopt-week      = "-" / (date-week ["-"])

   datespec-full     = datepart-fullyear date-month ["-"] date-mday
   datespec-year     = date-century / dateopt-century date-year
   datespec-month    = "-" dateopt-year date-month [["-"] date-mday]
   datespec-mday     = "--" dateopt-month date-mday
   datespec-week     = datepart-wkyear "W"
                       (date-week / dateopt-week date-wday)
   datespec-wday     = "---" date-wday
   datespec-yday     = dateopt-fullyear date-yday

   date              = datespec-full / datespec-year
                       / datespec-month /
   datespec-mday / datespec-week / datespec-wday / datespec-yday

Time:

   time-hour         = 2DIGIT ; 00-24
   time-minute       = 2DIGIT ; 00-59
   time-second       = 2DIGIT ; 00-58, 00-59, 00-60 based on
                              ; leap-second rules
   time-fraction     = ("," / ".") 1*DIGIT
   time-numoffset    = ("+" / "-") time-hour [[":"] time-minute]
   time-zone         = "Z" / time-numoffset

   timeopt-hour      = "-" / (time-hour [":"])
   timeopt-minute    = "-" / (time-minute [":"])

   timespec-hour     = time-hour [[":"] time-minute [[":"] time-second]]
   timespec-minute   = timeopt-hour time-minute [[":"] time-second]
   timespec-second   = "-" timeopt-minute time-second
   timespec-base     = timespec-hour / timespec-minute / timespec-second

   time              = timespec-base [time-fraction] [time-zone]

   iso-date-time     = date "T" time

Durations:

   dur-second        = 1*DIGIT "S"
   dur-minute        = 1*DIGIT "M" [dur-second]
   dur-hour          = 1*DIGIT "H" [dur-minute]
   dur-time          = "T" (dur-hour / dur-minute / dur-second)
   dur-day           = 1*DIGIT "D"
   dur-week          = 1*DIGIT "W"
   dur-month         = 1*DIGIT "M" [dur-day]
   dur-year          = 1*DIGIT "Y" [dur-month]
   dur-date          = (dur-day / dur-month / dur-year) [dur-time]

   duration          = "P" (dur-date / dur-time / dur-week)

Periods:

   period-explicit   = iso-date-time "/" iso-date-time
   period-start      = iso-date-time "/" duration
   period-end        = duration "/" iso-date-time

   period            = period-explicit / period-start / period-end

Appendix B. Day of the Week

The following is a sample C subroutine loosely based on Zeller's Congruence [ZELLER] which may be used to obtain the day of the week for dates on or after 0000-03-01:¶

   char *day_of_week(int day, int month, int year)
   {
      int cent;
      char *dayofweek[] = {
         "Sunday", "Monday", "Tuesday", "Wednesday",
         "Thursday", "Friday", "Saturday"
      };

      /* adjust months so February is the last one */
      month -= 2;
      if (month < 1) {
         month += 12;
         --year;
      }
      /* split by century */
      cent = year / 100;
      year %= 100;
      return (dayofweek[((26 * month - 2) / 10 + day + year
                        + year / 4 + cent / 4 + 5 * cent) % 7]);
   }

Appendix C. Leap Years

Here is a sample C subroutine to calculate if a year is a leap year:¶

   /* This returns non-zero if year is a leap year.  Must use 4 digit
      year.
    */
   int leap_year(int year)
   {
       return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
   }

Appendix D. Leap Seconds

Information about leap seconds can be found at: <http://tycho.usno.navy.mil/leapsec.html>. In particular, it notes that: ¶

The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to the CCIR Recommendation, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September.

When required, insertion of a leap second occurs as an extra second at the end of a day in UTC, represented by a timestamp of the form YYYY-MM-DDT23:59:60Z. A leap second occurs simultaneously in all time zones, so that time zone relationships are not affected. See Section 5.8 for some examples of leap second times.¶

The following table is an excerpt from the table maintained by the United States Naval Observatory. The source data is located at: ¶

<ftp://maia.usno.navy.mil/ser7/tai-utc.dat>

This table shows the date of the leap second, and the difference between the time standard TAI (which isn't adjusted by leap seconds) and UTC after that leap second.

UTC Date	TAI - UTC After Leap Second
1972-06-30	11
1972-12-31	12
1973-12-31	13
1974-12-31	14
1975-12-31	15
1976-12-31	16
1977-12-31	17
1978-12-31	18
1979-12-31	19
1981-06-30	20
1982-06-30	21
1983-06-30	22
1985-06-30	23
1987-12-31	24
1989-12-31	25
1990-12-31	26
1992-06-30	27
1993-06-30	28
1994-06-30	29
1995-12-31	30
1997-06-30	31
1998-12-31	32

Appendix E. Acknowledgements

The following people provided helpful advice for an earlier incarnation of this document: Ned Freed, Neal McBurnett, David Keegel, Markus Kuhn, Paul Eggert and Robert Elz. Thanks are also due to participants of the IETF Calendaring/Scheduling working group mailing list, and participants of the time zone mailing list.¶

The following reviewers contributed helpful suggestions for the present revision: Tom Harsch, Markus Kuhn, Pete Resnick, Dan Kohn. Paul Eggert provided many careful observations regarding the subtleties of leap seconds and time zone offsets. The following people noted corrections and improvements to earlier drafts: Dr John Stockton, Jutta Degener, Joe Abley, and Dan Wing.¶

Authors' Addresses

Graham Klyne (editor)
Clearswift Corporation
1310 Waterside
Arlington Business Park
Theale, Reading RG7 4SA
UK
Phone: +44 11 8903 8903
Fax: +44 11 8903 9000
EMail: GK@ACM.ORG

Chris Newman
Sun Microsystems
1050 Lakes Drive, Suite 250
West Covina, CA 91790
USA
EMail: chris.newman@sun.com

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