Using date and time formats in HTML - HTML | MDN

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Certain HTML elements use date and/or time values. The formats of the strings that specify these values are described in this article.
Elements that use such formats include certain forms of the <input> element that let the user choose or specify a date, time, or both, as well as the <ins> and <del> elements, whose datetime attribute specifies the date or date and time at which the insertion or deletion of content occurred.
For <input>, the type values of inputs whose value contains a string representing a date and/or time are:
Before getting into the intricacies of how date and time strings are written and parsed in HTML, here are some examples that should give you a good idea what the more commonly-used date and time string formats look like.
Before looking at the various formats of date and time related strings used by HTML elements, it is helpful to understand a few fundamental facts about the way they're defined. HTML uses a variation of the ISO 8601 standard for its date and time strings. It's worth reviewing the descriptions of the formats you're using in order to ensure that your strings are in fact compatible with HTML, as the HTML specification includes algorithms for parsing these strings that is actually more precise than ISO 8601, so there can be subtle differences in how date and time strings are expected to look.
Dates and times in HTML are always strings which use the ASCII character set.
In order to simplify the basic format used for date strings in HTML, the specification requires that all years be given using the modern (or proleptic) Gregorian calendar. While user interfaces may allow entry of dates using other calendars, the underlying value always uses the Gregorian calendar.
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.
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 contains 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:
All other years have 52 weeks.
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 generic month of the year. That is, rather than representing "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.
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.
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