Network Working Group C. Newman, Innosoft Internet Draft G. Klyne, Baltimore Technologies 3 April 2001 Expires: September 2001 Date and Time on the Internet: Timestamps Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Copyright Notice Copyright (C) The Internet Society 2001. All Rights Reserved. Abstract This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 [ISO8601] standard for representation of dates and times using the Gregorian calendar. Newman & Klyne FORMFEED[Page 1] Internet Draft Date and Time - Timestamps April 2001 Table of Contents 1. Introduction 2. Definitions 3. Two Digit Years 4. Local Time 4.1. Coordinated Universal Time (UTC) 4.2. Local Offsets 4.3. Unknown Local Offset Convention 4.4. Unqualified Local Time 5. Date and Time format 5.1. Ordering 5.2. Human Readability 5.3. Rarely Used Options 5.4. Redundant Information 5.5. Simplicity 5.6. Internet Date/Time Format 5.7. Restrictions 5.8. Examples 6. Acknowledgements 7. References 8. Security Considerations 9. Authors' Addresses Appendix A. ISO 8601 Collected ABNF Appendix B. Day of the Week Appendix C. Leap Years Appendix D. Leap Seconds Appendix E. Amendment history Full copyright statement 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 make 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. Newman & Klyne FORMFEED[Page 2] Internet Draft Date and Time - Timestamps April 2001 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: o All dates and times are assumed to be in the "current era", somewhere between 0AD and 9999AD. o 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.) o Date and time expressions indicate an instant in time. Description of time periods, or intervals, is not covered here. 2. Definitions UTC Coordinated Universal Time as maintained by the Bureau Internaational 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. 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 it shall be divisible by four hundred an integral number of times. Newman & Klyne FORMFEED[Page 3] Internet Draft Date and Time - Timestamps April 2001 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 822 [IMAIL] and amended by RFC 1123 [HOST-REQ]. Internet Date/Time Format The date format defined in section 5 of this document. 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: o Internet Protocols MUST generate four digit years in dates. o 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). o 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. o 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. Newman & Klyne FORMFEED[Page 4] Internet Draft Date and Time - Timestamps April 2001 4. Local Time 4.1. Coordinated Universal Time (UTC) Because the daylight rules for local timezones 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 timezone rules. 4.2. Local Offsets The offset between local time and UTC is often useful information. For example, in electronic mail [IMAIL] 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]. Therefore numeric offsets are now REQUIRED in Internet Mail Date/Time Format. 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 22:58:00Z. 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 semanticly from an offset of "Z" which implies that UTC is the preferred reference point for the specified time. This convention MAY also be used in the Email Date/Time Format. 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 timezone 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: o Use Network Time Protocol [NTP] to obtain the time in UTC. Newman & Klyne FORMFEED[Page 5] Internet Draft Date and Time - Timestamps April 2001 o Use another host in the same local timezone as a gateway to the Internet. This host MUST correct unqualified local times before they are transmitted to other hosts. o Prompt the user for the local timezone and daylight savings 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 timezones of the dates and times are the same (e.g. all in UTC), 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 analysers 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 Newman & Klyne FORMFEED[Page 6] Internet Draft Date and Time - Timestamps April 2001 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 redunant 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. Newman & Klyne FORMFEED[Page 7] Internet Draft Date and Time - Timestamps April 2001 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 ; 00-59, 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. 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. Newman & Klyne FORMFEED[Page 8] Internet Draft Date and Time - Timestamps April 2001 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 June (XXXX-06-30T23:59:60Z) or December (XXXX-12-31T23:59:60Z) if there is a leap second at that time (see Appendix D for a table of leap seconds). At all other times the maximum value of time-second is "59". Further, in timezones other than "Z", the leap second point is shifted by the zone offset (so it happens at the same instant around the globe). 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 three 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. Newman & Klyne FORMFEED[Page 9] Internet Draft Date and Time - Timestamps April 2001 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. 6. 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 timezone mailing list. 7. References [Zeller] Chr. Zeller, "Kalender-Formeln", Acta Mathematica, Vol. 9, Nov 1886. [IMAIL] Crocker, D., "Standard for the Format of Arpa Internet Text Messages", RFC 822, August 1982. [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [ISO8601] "Data elements and interchange formats -- Information interchange -- Representation of dates and times", ISO 8601:1988(E), International Organization for Standardization, June, 1988. [HOST-REQ] Braden, R., "Requirements for Internet Hosts -- Application and Support", RFC 1123, Internet Engineering Task Force, October 1989. Newman & Klyne FORMFEED[Page 10] Internet Draft Date and Time - Timestamps April 2001 [NTP] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation and Analysis", RFC 1305, University of Delaware, March 1992. [ITU-R-TF] International Telecommunication Union Recommendations for Time Signals and Frequency Standards Emissions. 8. 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. 9. Authors' Addresses Chris Newman Innosoft International, Inc. 1050 Lakes Drive West Covina, CA 91790 USA Email: chris.newman@innosoft.com Graham Klyne Baltimore Technologies - Content Security Group 1310 Waterside Arlington Business Park Theale Reading, RG7 4SA United Kingdom. Telephone: +44 118 903 8000 Facsimile: +44 118 903 9000 E-mail: GK@ACM.ORG Appendix A. ISO 8601 Collected ABNF 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 authoratative reference. Note that due to ambiguities in ISO 8601, some interpretations had to Newman & Klyne FORMFEED[Page 11] Internet Draft Date and Time - Timestamps April 2001 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 preceeded 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 Newman & Klyne FORMFEED[Page 12] Internet Draft Date and Time - Timestamps April 2001 Time: time-hour = 2DIGIT ; 00-24 time-minute = 2DIGIT ; 00-59 time-second = 2DIGIT ; 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 = date-time "/" date-time period-start = date-time "/" duration period-end = duration "/" date-time period = period-explicit / period-start / period-end Newman & Klyne FORMFEED[Page 13] Internet Draft Date and Time - Timestamps April 2001 Appendix B. Day of the Week The following is a sample C subroutine loosly based on Zeller's Congruence [Zeller] which may be used to obtain the day of the week: char *day_of_week(int day, int month, int year) { 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 - 2 * 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 This table is an excerpt from the table maintained by the United States Naval Observatory. The source data is located at: Newman & Klyne FORMFEED[Page 14] Internet Draft Date and Time - Timestamps April 2001 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 Appendix E. Amendment history 00a 30-Mar-2001 This document version created from Chris Newman's original 'draft-ietf-impp-datetime-00.txt'. Material relating to future times (schedule events) and timezone names has been removed. Added introductory text setting the scope for this document. Various small editorial changes. 00b 03-Apr-2001 Added reference [ABNF], and updated citations. Added comment about possible use of space-separated date/time fields. Added comment about possible use of lower case "t" and "z" in syntax. Corrected leap-second examples and noted that leap second point is offset by time zone. Newman & Klyne FORMFEED[Page 15] Internet Draft Date and Time - Timestamps April 2001 Full copyright statement Copyright (C) The Internet Society 2001. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Newman & Klyne FORMFEED[Page 16]