# IsoFields

Fields and units specific to the ISO-8601 calendar system, including quarter-of-year and week-based-year.

This class defines fields and units that are specific to the ISO calendar system.

### Quarter of year

The ISO-8601 standard is based on the standard civic 12 month year. This is commonly divided into four quarters, often abbreviated as Q1, Q2, Q3 and Q4.January, February and March are in Q1. April, May and June are in Q2. July, August and September are in Q3. October, November and December are in Q4.

The complete date is expressed using three fields:

- {@link #DAY_OF_QUARTER DAY_OF_QUARTER} - the day within the quarter, from 1 to 90, 91 or 92
- {@link #QUARTER_OF_YEAR QUARTER_OF_YEAR} - the week within the week-based-year
- {@link ChronoField#YEAR YEAR} - the standard ISO year

### Week based years

The ISO-8601 standard was originally intended as a data interchange format, defining a string format for dates and times. However, it also defines an alternate way of expressing the date, based on the concept of week-based-year.The date is expressed using three fields:

- {@link ChronoField#DAY_OF_WEEK DAY_OF_WEEK} - the standard field defining the day-of-week from Monday (1) to Sunday (7)
- #WEEK_OF_WEEK_BASED_YEAR - the week within the week-based-year
- {@link #WEEK_BASED_YEAR WEEK_BASED_YEAR} - the week-based-year

The week-based-year itself is defined relative to the standard ISO proleptic year. It differs from the standard year in that it always starts on a Monday.

The first week of a week-based-year is the first Monday-based week of the standard ISO year that has at least 4 days in the new year.

- If January 1st is Monday then week 1 starts on January 1st
- If January 1st is Tuesday then week 1 starts on December 31st of the previous standard year
- If January 1st is Wednesday then week 1 starts on December 30th of the previous standard year
- If January 1st is Thursday then week 1 starts on December 29th of the previous standard year
- If January 1st is Friday then week 1 starts on January 4th
- If January 1st is Saturday then week 1 starts on January 3rd
- If January 1st is Sunday then week 1 starts on January 2nd

There are 52 weeks in most week-based years, however on occasion there are 53 weeks.

For example:

<table cellpadding='0' cellspacing='3' border='0' style='text-align: left; width: 50%;'> <caption>Examples of Week based Years</caption> <tr><th>Date</th><th>Day-of-week</th><th>Field values</th></tr> <tr><th>2008-12-28</th><td>Sunday</td><td>Week 52 of week-based-year 2008</td></tr> <tr><th>2008-12-29</th><td>Monday</td><td>Week 1 of week-based-year 2009</td></tr> <tr><th>2008-12-31</th><td>Wednesday</td><td>Week 1 of week-based-year 2009</td></tr> <tr><th>2009-01-01</th><td>Thursday</td><td>Week 1 of week-based-year 2009</td></tr> <tr><th>2009-01-04</th><td>Sunday</td><td>Week 1 of week-based-year 2009</td></tr> <tr><th>2009-01-05</th><td>Monday</td><td>Week 2 of week-based-year 2009</td></tr> </table>

### Static properties of Class IsoFields

IsoFields.DAY_OF_QUARTER

The field that represents the day-of-quarter.

This field allows the day-of-quarter value to be queried and set. The day-of-quarter has values from 1 to 90 in Q1 of a standard year, from 1 to 91 in Q1 of a leap year, from 1 to 91 in Q2 and from 1 to 92 in Q3 and Q4.

The day-of-quarter can only be calculated if the day-of-year, month-of-year and year are available.

When setting this field, the value is allowed to be partially lenient, taking any value from 1 to 92. If the quarter has less than 92 days, then day 92, and potentially day 91, is in the following quarter.

IsoFields.QUARTER_OF_YEAR

The field that represents the quarter-of-year.

This field allows the quarter-of-year value to be queried and set. The quarter-of-year has values from 1 to 4.

The day-of-quarter can only be calculated if the month-of-year is available.

IsoFields.WEEK_OF_WEEK_BASED_YEAR

The field that represents the week-of-week-based-year.

This field allows the week of the week-based-year value to be queried and set.

IsoFields.WEEK_BASED_YEAR

The field that represents the week-based-year.

This field allows the week-based-year value to be queried and set.

IsoFields.WEEK_BASED_YEARS

The unit that represents week-based-years for the purpose of addition and subtraction.

This allows a number of week-based-years to be added to, or subtracted from, a date. The unit is equal to either 52 or 53 weeks. The estimated duration of a week-based-year is the same as that of a standard ISO year at {@code 365.2425 Days}.

The rules for addition add the number of week-based-years to the existing value for the week-based-year field. If the resulting week-based-year only has 52 weeks, then the date will be in week 1 of the following week-based-year.

IsoFields.QUARTER_YEARS

Unit that represents the concept of a quarter-year. For the ISO calendar system, it is equal to 3 months. The estimated duration of a quarter-year is one quarter of {@code 365.2425 Days}.