Why does java.time.Period#normalized() not normalize days?

Question

In the Java class java.time.Period the method normalized() has the following in its Javadoc:

This normalizes the years and months units, leaving the days unit unchanged.

The superclass' method has the following in its Javadoc:

The process of normalization is specific to each calendar system. For example, in the ISO calendar system, the years and months are normalized but the days are not, [...]

I do not have access to the actual text of ISO 8601-1:2019, and would not like to spend hundreds of [insert currency here]s on it (my guess is that normalization may be described in Part 1: Basic rules and not in Part 2: Extensions).

Could someone shed light upon why Period#normalized() does not normalize days? Does it really come directly from ISO 8601 itself, is it somewhere else specified, or is it just specific to the Java implementation?

Answer 1

This is because a period of years or months is always the same amount of time (the same period) for any given date. A year is always 12 months, 12 months are always a year, thus these parts of the period can easily be normalized.

However days are variable in relation to months and years. If you have a period of 1 year, 1 month and 32 days, you cannot normalize this to 1 year, 2 months and then a fixed amount of days, because it might be 1 day, 2 days, 3 days or 4 days, depending on which date you will apply the period on.

A month can be 28, 29, 30 or 31 days. A year can be 365 or 366 days. And since a period is independent of any fixed date, there is no way to decide these relations.

Example:

2019-01-01 + 01-01-32 is 2020-03-04

2020-01-01 + 01-01-32 is 2021-03-03

2020-02-01 + 01-01-32 is 2021-04-02

2020-03-01 + 01-01-32 is 2021-05-03

As you can see the days resulting from applying the same period to different dates varies depending on the month and on if it's a leap year.

Thus it is impossible to normalize days in a period and the days are not touched when normalizing.