The hyphens can be omitted if compactness of the representation is more important than human readability, for example as in 19950204. For situations where information about the century is really not required, a two-digit year representation is available: 95-02-04 or 950204.

If only the month or even only the year is of interest: 1995-02 or 1995

In commercial and industrial applications, especially in Europe, it is often required to refer to a week of a year. Week 01 of a year is by definition the first week that has the Thursday in this year, which is equivalent to the week that contains the fourth day of January. In other words, the first week of a new year is the week that has the majority of its days in the new year. Week 01 might also contain days from the previous year and the week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year. A week starts with Monday (day 1) and ends with Sunday (day 7). For example, the first week of the year 1997 lasts from 1996-12-30 to 1997-01-05 and can be written in standard notation as 1997-W01 or 1997W01

The week notation can also be extended by a number indicating the day of the week. For example, the day 1996-12-31, which is the Tuesday (day 2) of the first week of 1997, can also be written as 1997-W01-2 or 1997W012 for applications like industrial planning where many things like shift rotations are organized per week and knowing the week number and the day of the week is more handy than knowing the day of the month. An abbreviated version of the year and week number like 95W05 is sometimes useful as a compact code printed on a product that indicates when it has been manufactured.

Time of Day

The international standard notation for the time of day is hh:mm:ss where hh is the number of complete hours that have passed since midnight (00-24), mm is the number of complete minutes that have passed since the start of the hour (00-59), and ss is the number of complete seconds since the start of the minute (00-59). If the hour value is 24, then the minute and second values must be zero. 23:59:59 which represents the time one second before midnight. As with the date notation, the separating colons can also be omitted as in 235959 and the precision can be reduced by omitting the seconds or both the seconds and minutes as in 23:59, 2359, or 23 It is also possible to add fractions of a second after a decimal dot or comma, for instance the time 5.8 ms before midnight can be written as 23:59:59.9942 or 235959.9942

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04 24:00 = 1995-02-05 00:00 In case an unambiguous representation of time is required, 00:00 is usually the preferred notation for midnight and not 24:00. Digital clocks display 00:00 and not 24:00. If a date and a time are displayed on the same line, then always write the date in front of the time. If a date and a time value are stored together in a single data field, then ISO 8601 suggests that they should be separated by a latin capital letter T, as in 19951231T235959.

A remark for readers from the U.S.: The 24h time notation specified here has already been the de-facto standard all over the world in written language for decades. The common 24h international standard notation starts to get widely used now even in England. Most other languages don't even have abbreviations like "a.m." and "p.m." and the 12h notation is hardly used on Continental Europe to write or display a time. Even in the U.S., the military and computer programmers have been using the 24h notation for a long time. Avoid using the 12h notation, especially in technical applications! Even in the U.S., the widely respected Chicago Manual of Style recommends now to use the international standard time notation in publications.

A remark for readers from German speaking countries: In May 1996, the German standard DIN 5008, which specifies typographical rules for German texts written on typewriters, has been updated. The old German numeric date notations DD.MM.YYYY and DD.MM.YY have been replaced by the ISO date notations YYYY-MM-DD and YY-MM-DD. Similarly, the old German time notations and have been replaced by the ISO notations hh:mm and hh:mm:ss. Those new notations are now also mentioned in the latest edition of the Duden. The German alphanumeric date notation continues to be for example "3. August 1994" or "3. Aug. 1994".

The corresponding Austrian standard has already used the ISO 8601 date and time notations before. ISO 8601 has been adopted as European Standard EN 28601 and is therefore now a valid standard in all EU countries and all conflicting national standards have been changed accordingly.

Time Zone Without any further additions, a date and time as written above is assumed to be in some local time zone. In order to indicate that a time is measured in Universal Time (UTC), you can append a capital letter Z to a time as in 23:59:59Z or 2359Z [The Z stands for the "zero meridian", which goes through Greenwich in London, and it is also commonly used in radio communication where it is pronounced "Zulu" (the word for Z in the international radio alphabet). Universal Time (sometimes also called "Zulu Time") was called Greenwich Mean Time (GMT) before 1972, however this term should no longer be used. Since the introduction of an international atomic time scale, almost all existing civil time zones are now related to UTC, which is slightly different from the old and now unused GMT.] The strings +hh:mm, +hhmm, or +hh can be added to the time to indicate that the used local time zone is hh hours and mm minutes ahead of UTC. For time zones west of the zero meridian, which are behind UTC, the notation -hh:mm, -hhmm, or -hh is used instead. For example, Central European Time (CET) is +0100 and U.S./Canadian Eastern Standard Time (EST) is -0500.

The following strings all indicate the same point of time: 12:00Z = 13:00+01:00 = 0700-0500 There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. In addition, politicians enjoy modifying the rules for civil time zones, especially for daylight saving times, every few years, so the only really reliable way of describing a local time zone is to specify numerically the difference of local time to UTC. Better use directly UTC as your only time zone where this is possible and then you do not have to worry about time zones and daylight saving time changes at all.

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