Especially authors of Web pages and software engineers who design user interfaces, file formats, and communication protocols should be familiar with ISO 8601.Contents: Date, time of day, time zone, software hints.Possible ISO week numbers are in the range 01 to 53. (There is one historic exception: the year in which the Gregorian calendar was introduced had less than 365 days and less than 52 weeks.) Proof: Per definition, the first week of a year is W01 and consequently days before week W01 belong to the previous year and so there is no week with lower numbers.Considering the highest possible week number, the worst case is a leap year like 1976 that starts with a Thursday, because this keeps the highest possible number of days of W01 in the previous year, i.e. In this case, the Sunday of W52 of the worst case year is day number 4 51*7=361 and 361-366=5 days of W53 belong still to this year, which guarantees that in the worst case year day 4 (Thursday) of W53 is not yet in the next year, so a week number 53 is possible.
Apart from the recommended primary standard notation YYYY-MM-DD, ISO 8601 also specifies a number of alternative formats for use in applications with special requirements.Note: The value 60 for ss might sometimes be needed during an inserted leap second in an atomic time scale like Coordinated Universal Time (UTC).A single leap second is inserted into the UTC time scale every few years as announced by the International Earth Rotation Service in Paris, to keep UTC from wandering away more than 0.9 s from the less constant astronomical time scale UT1, which is defined by the actual rotation of the earth.In this case, the Sunday of week 52 would be day number 3 52*7=367, i.e.only the last 367-365=2 days of the W52 reach into the next year and consequently, even a worst case year like 1999 has a week W52 including the days 1999-12-27 to 2000-01-02. Both day and year are useful units of structuring time, because the position of the sun on the sky, which influences our lives, is described by them.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 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.The international standard date notation is where YYYY is the year in the usual Gregorian calendar, MM is the month of the year between 01 (January) and 12 (December), and DD is the day of the month between 01 and 31.For example, the fourth day of February in the year 1995 is written in the standard notation as Other commonly used notations are e.g.All of these alternatives can easily and automatically be distinguished from each other: The hyphens can be omitted if compactness of the representation is more important than human readability, for example as in In commercial and industrial applications (delivery times, production plans, etc.), especially in Europe, it is often required to refer to a week of a year.Week 01 of a year is per definition the first week that has the Thursday in this year, which is equivalent to the week that contains the fourth day of January.