日期和时间的表示格式，有专门的标准，这里介绍ISO 8601。
内容主要是来自维基百科。

1、简单介绍

ISO 8601 is an international standard covering the worldwide exchange and communication of date and time related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019. The standard aims to provide a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times.

ISO 8601是全世界日期和时间相关的数据交换的国际标准。这个标准的目标是在全世界范围的通信中提供格式良好的、无歧义的时间和日期表示。

In general, ISO 8601 applies to these representations and formats: dates, in the Gregorian calendar (including the proleptic Gregorian calendar); times, based on the 24-hour timekeeping system, with optional UTC offset; time intervals; and combinations thereof. The standard does not assign specific meaning to any element of the dates/times represented: the meaning of any element depends on the context of its use. Dates and times represented cannot use words that do not have a specified numerical meaning within the standard (thus excluding names of years in the Chinese calendar), or that do not use computer characters (excludes images or sounds).

总体来说，ISO 8601涵盖了日期、时间(24小时格式，包含UTC时差)、时区、以及它们组合的表示描述。在这个标准中，日期和时间都不能够使用没有数字含义的单词表示(这样的话，中国农历中的生肖年是没法表示的)。

In representations that adhere to the ISO 8601 interchange standard, dates and times are arranged such that the greatest temporal term (typically a year) is placed at the left and each successively lesser term is placed to the right of the previous term. Representations must be written in a combination of Arabic numerals and the specific computer characters (such as "-", ":", "T", "W", "Z") that are assigned specific meanings within the standard; that is, such commonplace descriptors of dates (or parts of dates) as "January", "Thursday", or "New Year's Day" are not allowed in interchange representations within the standard.

在ISO 8601的表示中，日期和时间按照这样的顺序排列：大的单位(比如年)放在左边，小的单位依次往右排列。表示只能由阿拉伯数字和指定的特殊字符(比如"-", ":", "T", "W", "Z")等组成，这样的话，日常常用的描述性的单词(比如"January", "Thursday", or "New Year's Day")是不允许使用的。

2、历史

The first edition of the ISO 8601 standard was published as ISO 8601:1988 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. 2000, by a third edition ISO 8601:2004 published on 1 December 2004, and withdrawn and revised by ISO 8601-1:2019 and ISO 8601-2:2019 on 25 February 2019. ISO 8601 was prepared by, It has been superseded by a second edition ISO 8601:2000 in 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.
Issued in February 2019, the fourth revision of the standard ISO 8601-1:2019 represents slightly updated contents of the previous ISO 8601:2004 standard, (EDTF). whereas the new ISOISO 8601-2:2019 defines various extensions such as uncertainties or parts of the Extended Date/Time Format(EDTF).

3、主要原则

General principles
Date and time values are ordered from the largest to smallest unit of time: 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 or time offset. 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.
(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 most to the least 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.

4、日期

The standard uses the Gregorian calendar, which "serves as an international standard for civil use."
ISO 8601:2004 fixes a reference calendar date to the Gregorian calendar of 20 May 1875 as the date the Convention du Mètre (Metre Convention) was signed in Paris (the explicit reference date was removed in ISO 8601-1:2019). However, ISO calendar dates before the convention are still compatible with the Gregorian calendar all the way back to the official introduction of the Gregorian calendar on 15 October 1582.
Earlier dates, in the proleptic Gregorian calendar, may be used by mutual agreement of the partners exchanging information. The standard states that every date must be consecutive, so usage of the Julian calendar would be contrary to the standard (because at the switchover date, the dates would not be consecutive).

4.1 年

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 before 1583 are not automatically allowed by the standard. Instead "values in the range [0000] through [1582] 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 more common AD/BC(or CE/BCE) notation; by convention 1 BC is labelled +0000, 2 BC is labeled −0001, and so on.
公元前BC（Before Christ）意思是基督之前；公元后AD（Anno Domini）意思是耶稣基督之年，乃拉丁文缩写版（anno Domini nostri Jesu Christi）。

整体用4位数字表示年份，如果要表示公元前的年份，需要在四位数字前面再加一个正负号表示。格式如下：

YYYY
±YYYYY

4.2 日历日期

Calendar date representations are in the form shown in the adjacent box. [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"[14] 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". The 2000 version allowed writing "--04-05" to mean "April 5" but the 2004 version does not allow omitting the year when a month is present. One may simply write "1981" to refer to that year, "198" to refer to the decade from 1980 to 1989 inclusive, 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).

格式如下：

YYYY-MM-DD or YYYYMMDD
YYYY-MM (but not YYYYMM)
Only allowed in the superseded version from
2000:
--MM-DD or --MMDD[1]

4.3 周数日期

Week date representations are in the formats as shown in the adjacent box. [YYYY] indicates the ISO weeknumbering 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.
There are several mutually equivalent and compatible descriptions of week 01:

• the week with the first business day in the starting year (considering that Saturdays, Sundays and 1st January are non-working days),
• the week with the starting 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.

As a consequence, 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 first ISO week of a year may have up to three days that are actually in the Gregorian calendar year that is ending; if three, they are Monday, Tuesday and Wednesday. Similarly, the last ISO week of a year may have up to three days that are actually in the Gregorian calendar year that is starting; if three, they are Friday, Saturday, and Sunday. The Thursday of each ISO week is always in the Gregorian calendar year denoted by the ISO week-numbering year.
Examples:
Monday 29 December 2008 is written "2009-W01-1"
Sunday 3 January 2010 is written "2009-W53-7"

格式：

YYYY-Www or YYYYWww
YYYY-Www-D or YYYYWwwD

4.4 日数日期

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 referred to as "Julian Date", but this can cause confusion with the astronomical Julian day, 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 0000).

格式：

YYYY-DDD or YYYYDDD

5、时间

5.1 时间格式简介

ISO 8601 uses the 24-hour clock system. As of ISO 8601-1:2019, the basic format is T[hh][mm][ss] and the extended format is T[hh]:[mm]:[ss]. Earlier versions omitted the T (representing time) in both formats.

• [hh] refers to a zero-padded hour between 00 and 23.
• [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 "T134730" in the basic format or "T13:47:30" in the extended format. ISO 8601-1:2019 allows the T to be omitted in the extended format, as in "13:47:30", but only allows the T to be omitted in the basic format when there is no risk of ambiguity with date expressions.
Either the seconds, or the minutes and seconds, may be omitted from the basic or extended time formats for greater brevity but decreased precision; the resulting reduced precision time formats are:

• T[hh][mm] in basic format or [hh]:[mm] in extended format, when seconds are omitted.
• T[hh], when both seconds and minutes are omitted.

As of ISO 8601-1:2019 midnight may only be referred to as "00:00", corresponding to the beginning of a calendar day. Earlier versions of the standard allowed "24:00" corresponding to the end of a day, but this is explicitly disallowed by the 2019 revision.
A decimal fraction may be added to the lowest order time element present, in any of these representations. A decimal mark, either a comma or a dot (following ISO 80000-1 according to ISO 8601:1-2019,[25] which does not stipulate a preference except within International Standards, but with a preference for a comma according to ISO 8601:2004)[26] 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", "T1430,5", "14:30.5", or "T1430.5". 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. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3 for a DATETIME, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".

格式为：

hh:mm:ss.sss or Thhmmss.sss
hh:mm:ss or Thhmmss
hh:mm or Thhmm
Thh

5.2 时区修饰符

Time zones in ISO 8601 are represented as local time (with the location unspecified), as UTC, or as an offset from UTC.

时区修饰符有三种形式：什么都不加，表示本地时间；加Z后缀表示UTC时间；也可以加一个标识和UTC时间的时区偏移。格式如下：

本地时间：
<time>
UTC时间：
<time>Z
UTC时区偏移时间：
<time>±hh:mm
<time>±hhmm
<time>±hh

5.2.1 本地时间（Local time (unqualified)）

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. Even within a single geographic time zone, some local times will be ambiguous if the region observes daylight saving time. It is usually preferable to indicate a time zone (zone designator) using the standard's notation.

5.2.2 UTC时间（Coordinated Universal Time (UTC)）

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 "T0930Z". "14:45:15 UTC" would be "14:45:15Z" or "T144515Z".
The Z suffix in the ISO 8601 time representation is sometimes referred to as "Zulu time" because the same letter is used to designate the Zulu time zone. However the ACP 121 standard that defines the list of military time zones makes no mention of UTC and derives the "Zulu time" from the Greenwich Mean Time which was formerly used as the international civil time standard. GMT is no longer precisely defined by the scientific community and can refer to either UTC or UT1 depending on context.

5.2.3 和UTC的时区偏移时间（Time offsets from UTC）

The UTC offset is appended to the time in the same way that 'Z' was above, in the form ±[hh]:[mm], ±[hh][mm], or ±[hh].
Negative UTC offsets describe a time zone west of UTC±00:00, where the civil time is behind (or earlier) than UTC so the zone designator will look like "−03:00","−0300", or "−03".
Positive UTC offsets describe a time zone at or east of UTC±00:00, where the civil time is the same as or ahead (or later) than UTC so the zone designator will look like "+02:00","+0200", or "+02".
Examples

• "−05:00" for New York on standard time (UTC-05:00)
• "−04:00" for New York on daylight saving time (UTC-04:00)
• "+00:00" (but not "−00:00") for London on standard time (UTC±00:00)
• "+02:00" for Cairo (UTC+02:00)
• "+05:30" for Mumbai (UTC+05:30)
• "+14:00" for Kiribati (UTC+14:00)

See List of UTC time offsets for other UTC offsets.
To represent a negative offset, ISO 8601 specifies using a minus sign. If the interchange character set is limited and does not have a minus sign character, then the hyphen-minus should be used. ASCII does not have a minus sign, so its hyphen-minus character (code is 45 decimal or 2D hexadecimal) would be used. If the character set has a minus sign, then that character should be used. Unicode has a minus sign, and its character code is U+2212 (2212 hexadecimal); the HTML character entity invocation is − .
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[30] 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.

6、日期和时间组合

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". In ISO 8601:2004 it was permitted to omit the "T" character by mutual agreement as in "200704051430", but this provision was removed in ISO 8601-1:2019. Separating date and time parts with other characters such as space is not allowed in ISO 8601, but allowed in its profile RFC 3339.
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 may be represented using a specified reduced precision format.

时间和日期的组合可以表示为：

<date>T<time>

7、时间长度

Durations define the amount of intervening time in a time interval and are represented by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S or P[n]W as shown on the aside. 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 (for 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. However, at least one element must be present, thus "P" is not a valid representation for a duration of 0 seconds. "PT0S" or "P0D", however, are both valid and represent the same duration.
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,[35] 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. But keep in mind that "PT36H" is not the same as "P1DT12H" when switching from or to Daylight saving time.
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).
Although the standard describes a duration as part of time intervals, which are discussed in the next section, the duration format (or a subset thereof) is widely used independent of time intervals, as with the Java 8 Duration class.

格式：

PnYnMnDTnHnMnS
PnW
P<date>T<time>

8、时间区间

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:

1. Start and end, such as "2007-03-01T13:00:00Z/2008-05-11T15:30:00Z"
2. Start and duration, such as "2007-03-01T13:00:00Z/P1Y2M10DT2H30M"
3. Duration and end, such as "P1Y2M10DT2H30M/2008-05-11T15:30:00Z"
4. 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 (more commonly referred to as a forward slash "/"). Section 3.2.6 of ISO 8601-1:2019 notes that "A solidus may be replaced by a double hyphen ["--"] by mutual agreement of the communicating partners", and previous versions used notations like "2000--2002". Use of a double hyphen instead of a solidus allows inclusion in computer filenames; in common operating systems, a solidus is a reserved character and is not allowed in a filename.
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 "2007-11-13T00:00/16T00:00".

格式：

<start>/<end>
<start>/<duration>
<duration>/<end>
<duration>

8.1 重复时间区间

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] or specifying a value of -1, means an unbounded number of repetitions. A value of 0 for [n] means the interval is not repeated.
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".

格式：

Rn/<interval>
R/<interval>

参考资料

• 维基百科：ISO 8601