HTML で使われる日付や時刻の形式

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一部の HTML 要素は日付や時刻の値を使用します。この記事ではこれらの値を指定する文字列の形式について説明しています。このような形式を使用する要素には、ユーザーに日付、時刻、またはその両方を選択または指定させる <input> 要素の一部の形、同様に <ins> および <del> 要素で、 datetime 属性によって内容の挿入または削除が行われた日付や日時を指定しているものなどがあります。

<input> については、 type の値で日付や時刻を表す文字列を含む value を返すものは次の通りです。

日付と時刻の文字列が HTML でどのように記述され解析されるかの複雑さを理解する前に、より一般的に使用される日付と時刻の文字列形式がどのように見えるかを理解するための例をいくつか示します。

HTML の日付と時刻の文字列の例
文字列 日付または時刻
2005-06-07 2005年6月7日 [詳細]
08:45 午前8時45分 [詳細]
08:45:25 午前8時45分25秒 [詳細]
0033-08-04T03:40 33年8月4日午前3時40分 [詳細]
1977-04-01T14:00:30 1977年4月1日午後2時30秒 [詳細]
1901-01-01T00:00Z UTC での1901年1月1日の夜半 [詳細]
1901-01-01T00:00:01-04:00 米東部標準時の1901年1月1日の夜半から1秒後 [詳細]


HTML 要素で使用される文字列に関する日付や時刻のさまざまな形式を見てみる前に、これらの定義方法についての基本的な事実をいくつか理解してしておくと役に立ちます。 HTML では、日付や時刻の文字列に ISO 8601 標準の一種を使用します。 HTML 仕様書には、実際に ISO 8601 よりも詳細に文字列を解析するアルゴリズムが含まれており、日付と時刻の外見について微妙な違いがある可能性があるので、文字列が実際に HTML と互換性があることを確認するためには、使用している形式の説明を確認する価値があります。


HTML において、日付と時刻は常に ASCII 文字セットを使用した文字列です。

HTML で日付の文字列に使われる基本的な形式を単純化するために、仕様書ではすべての年をグレゴリオ暦 (または先発グレゴリオ暦) を使用して指定することを要求しています。ユーザーインターフェイスは他のカレンダーを使用した日付を入力することもできますが、元となる値は常にグレゴリオ暦を使用します。

While the Gregorian calendar wasn't created until the year 1582 (replacing the similar Julian calendar), for HTML's purposes, the Gregorian calendar is extended back to the year 1 C.E. Make sure any older dates account for this.

For the purposes of HTML dates, years are always at least four digits long; years prior to the year 1000 are padded with leading zeroes ("0"), so the year 72 is written as 0072. Years prior to the year 1 C.E. are not supported, so HTML doesn't support years 1 B.C.E. (1 B.C.) or earlier.

A year is normally 365 days long, except during leap years.


A leap year is any year which is divisible by 400 or the year is divisible by 4 but not by 100. Although the calendar year is normally 365 days long, it actually takes the planet Earth approximately 365.2422 days to complete a single orbit around the sun. Leap years help to adjust the calendar to keep it synchronized with the actual position of the planet in its orbit. Adding a day to the year every four years essentially makes the average year 365.25 days long, which is close to correct.

The adjustments to the algorithm (taking a leap year when the year can be divided by 400, and skipping leap years when the year is divisible by 100) help to bring the average even closer to the correct number of days (365.2425 days). Scientists occasionally add leap seconds to the calendar (seriously) to handle the remaining three ten-thousandths of a day and to compensate for the gradual, naturally occurring slowing of Earth's rotation.

While month 02, February, normally has 28 days, it has 29 days in leap years.

There are 12 months in the year, numbered 1 through 12. They are always represented by a two-digit ASCII string whose value ranges from 01 through 12. See the table in the section Days of the month for the month numbers and their corresponding names (and lengths in days).

Month numbers 1, 3, 5, 7, 8, 10, and 12 are 31 days long. Months 4, 6, 9, and 11 are 30 days long. Month 2, February, is 28 days long most years, but is 29 days long in leap years. This is detailed in the following table.

The months of the year and their lengths in days
Month number Name (English) Length in days
01 January 31
02 February 28 (29 in leap years)
03 March 31
04 April 30
05 May 31
06 June 30
07 July 31
o8 August 31
09 September 30
10 October 31
11 November 30
12 December 31


A week string specifies a week within a particular year. A valid week string consists of a valid year number, followed by a hyphen character ("-", or U+002D), then the capital letter "W" (U+0057), followed by a two-digit week of the year value.

The week of the year is a two-digit string between 01 and 53. Each week begins on Monday and ends on Sunday. That means it's possible for the first few days of January to be considered part of the previous week-year, and for the last few days of December to be considered part of the following week-year. The first week of the year is the week that conains the first Thursday of the year. For example, the first Thursday of 1953 was on January 1, so that week—beginning on Monday, December 29—is considered the first week of the year. Therefore, December 30, 1952 occurs during the week 1953-W01.

A year has 53 weeks if:

  • The first day of the calendar year (January 1) is a Thursday or
  • The first day of the year (January 1) is a Wednesday and the year is a leap year

All other years have 52 weeks.

Examples of valid week strings
Week string Week and year (Date range)
2001-W37 Week 37, 2001 (September 10-16, 2001)
1953-W01 Week 1, 1953 (December 29, 1952-January 4, 1953)
1948-W53 Week 53, 1948 (December 27, 1948-January 2, 1949)
1949-W01 Week 1, 1949 (January 3-9, 1949)
0531-W16 Week 16, 531 (April 13-19, 531)
0042-W04 Week 4, 42 (January 21-27, 42)

Note that both the year and week numbers are padded with leading zeroes, with the year padded to four digits and the week to two.


A month string represents a specific month in time, rather than a genetic month of the year. That is, rather than representing simply "January," an HTML month string represents a month and year paired, like "January 1972."

A valid month string consists of a valid year number (a string of at least four digits), followed by a hyphen character ("-", or U+002D), followed by a two-digit numeric month number, where 01 represents January and 12 represents December.

月の文字列 年月
17310-09 September, 17310
2019-01 January, 2019
1993-11 November, 1993
0571-04 April, 571
0001-07 July, 1 C.E.

Notice that all years are at least four characters long; years that are fewer than four digits long are padded with leading zeroes.


A valid date string consists of a month string, followed by a hyphen character ("-", or U+002D), followed by a two-digit day of the month.

月の文字列 日付全体
1993-11-01 November 1, 1993
1066-10-14 October 14, 1066
0571-04-22 April 22, 571
0062-02-05 February 5, 62


A time string can specify a time with precision to the minute, second, or to the millisecond. Specifying only the hour or minute isn't permitted. A valid time string minimally consists of a two-digit hour followed by a colon (":", U+003A), then a two-digit minute. The minute may optionally be followed by another colon and a two-digit number of seconds. Milliseconds may be specified, optionally, by adding a decimal point character (".", U+002E) followed by one, two, or three digits.

There are some additional basic rules:

  • The hour is always specified using the 24-hour clock, with 00 being midnight and 11 PM being 23. No values outside the range 0023 are permitted.
  • The minute must be a two-digit number between 00 and 59. No values outside that range are allowed.
  • If the number of seconds is omitted (to specify a time accurate only to the minute), no colon should follow the number of minutes.
  • If specified, the integer portion of the number of seconds must be between 00 and 59. You cannot specify leap seconds by using values like 60 or 61.
  • If the number of seconds is specified and is an integer, it must not be followed by a decimal point.
  • If a fraction of a second is included, it may be from one to three digits long, indicating the number of milliseconds. It follows the decimal point placed after the seconds component of the time string.
時刻の文字列 時刻
00:00:30.75 12:00:30.75 AM (30.75 seconds after midnight)
12:15 12:15 PM
13:44:25 1:44:25 PM (25 seconds after 1:44 PM)

Local date and time strings

A valid datetime-local string consists of a date string and a time string concatenated together with either the letter "T" or a space character separating them. No information about the time zone is included in the string; the date and time is presumed to be in the user's local time zone.

When you set the value of a datetime-local input, the string is normalized into a standard form. Normalized datetime strings always use the letter "T" to separate the date and the time, and the time portion of the string is as short as possible. This is done by leaving out the seconds component if its value is :00.

Examples of valid datetime-local strings
Date/time string Normalized date/time string Actual date and time
1986-01-28T11:38:00.01 1986-01-28T11:38:00.01 January 28, 1986 at 11:38:00.01 AM
1986-01-28 11:38:00.010 1986-01-28T11:38:00.011 January 28, 1986 at 11:38:00.01 AM
0170-07-31T22:00:00 0170-07-31T22:002 July 31, 170 at 10:00 PM
  1. Notice that after normalization, this is the same string as the previous datetime-local string. The space has been replaced with the "T" character and the trailing zero in the fraction of a second has been removed to make the string as short as possible.
  2. Note that the normalized form of this date drops the ":00" indicating the number of seconds to be zero, because the seconds are optional when zero, and the normalized string minimizes the length of the string.

Global date and time strings

A global date and time string specifies a date and time as well as the time zone in which it occurs. A valid global date and time string is the same format as a local date and time string, except it has a time zone string appended to the end, following the time.

Time zone offset string

A time zone offset string specifies the offset in either a positive or a negative number of hours and minutes from the standard time base. There are two standard time bases, which are very close to the same, but not exactly the same:

  • For dates after the establishment of Coordinated Universal Time (UTC) in the early 1960s, the time base is Z and the offset indicates a particular time zone's offset from the time at the prime meridian at 0º longitude (which passes through the Royal Observatory at Greenwich, England).
  • For dates prior to UTC, the time base is instead expressed in terms of UT1, which is the contemporary Earth solar time at the prime meridian.

The time zone string is appended immediately following the time in the date and time string. You can specify simply "Z" as the time zone offset string to indicate that the time is specified in UTC. Otherwise, the time zone string is constructed as follows:

  1. A character indicating the sign of the offset: the plus character ("+", or U+002B) for time zones to the east of the prime meridian or the minus character ("-", or U+002D) for time zones to the west of the prime meridian.
  2. A two-digit number of hours that the time zone is offset from the prime meridian. This value must be between 00 and 23.
  3. An optional colon (":") character.
  4. A two-digit number of minutes past the hour; this value must be between 00 and 59.

While this format allows for time zones between -23:59 and +23:59, the current range of time zone offsets is -12:00 to +14:00, and no time zones are currently offset from the hour by anything other than 00, 30, or 45 minutes. This may change at more or less anytime, since countries are free to tamper with their time zones at any time and in any way they wish to do so.

Examples of valid global date and time strings
Global date and time string Actual global date and time Date and time at prime meridian
2005-06-07T00:00Z June 7, 2005 at midnight UTC June 7, 2005 at midnight
1789-08-22T12:30:00.1-04:00 August 22, 1789 at a tenth of a second past 12:30 PM Eastern Daylight Time (EDT) August 22, 1789 at a tenth of a second past 4:30 PM
3755-01-01 00:00+10:00 January 1, 3755 at midnight Australian Eastern Standard Time (AEST) December 31, 3754 at 2:00 PM