ISO week year numbering therefore usually deviates by 1 from the Gregorian for some days close to 1 January.
A precise date is specified by the ISO week-numbering year in the format YYYY, a week number in the format ww prefixed by the letter 'W', and the weekday number, a digit d from 1 through 7, beginning with Monday and ending with Sunday.
The ISO year is slightly offset to the Gregorian year; for example, Monday 30 December 2019 in the Gregorian calendar is the first day of week 1 of 2020 in the ISO calendar, and is written as 2020-W01-1 or 2020W011.
The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday: If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in W01.
The ISO standard does not define any association of weeks to months.
A date is either expressed with a month and day-of-the-month, or with a week and day-of-the-week, never a mix.
Weeks are a prominent entity in accounting where annual statistics benefit from regularity throughout the years.
Therefore, a fixed length of 13 weeks per quarter is usually chosen in practice.
The 5-week months would meet one of the following three criteria: For all years, 8 days have a fixed ISO week number (between W01 and W08) in January and February.
With the exception of leap years starting on Thursday, dates with fixed week numbers occur in all months of the year (for 1 day of each ISO week W01 to W52).
Solar astronomic phenomena, such as equinoxes and solstices, vary in the Gregorian calendar over a range spanning three days, over the course of each 400-year cycle, while the ISO Week Date calendar has a range spanning 9 days.
A programming bug confusing these two year numbers is probably the cause of some Android users of Twitter being unable to log in around midnight of 29 December 2014 UTC.
As a result, extra weeks are spread across the 400-year cycle in a complex, seemingly random pattern.
Most calendar reform proposals using leap week designs strive to simplify and harmonize this pattern, some by choosing a different leap cycle (e.g. 293 years).
Not all parts of the world consider the week to begin with Monday.
Correspondence of lexicographical order and chronological order is preserved (just like with the ISO year-week-weekday numbering), but partial weeks make some computations of weekly statistics or payments inaccurate at the end of December or the beginning of January or both.
Up to six days of the previous December may be part of the first week of the year.