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

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8

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?

java datetime java-time iso8601 period

share|improve this question

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Zabuza: I'm familiar with the Wikipedia page, however, it does not cover normalization of dates/periods.
  
  – D. Kovács
  8 hours ago
  

  
  
  
  

add a comment | 

8

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?

java datetime java-time iso8601 period

share|improve this question

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Zabuza: I'm familiar with the Wikipedia page, however, it does not cover normalization of dates/periods.
  
  – D. Kovács
  8 hours ago
  

  
  
  
  

add a comment | 

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?

java datetime java-time iso8601 period

share|improve this question

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

share|improve this question

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

share|improve this question

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

edited 7 hours ago

Basil Bourque

127k3636 gold badges427427 silver badges600600 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

asked 9 hours ago

D. KovácsD. Kovács

82555 silver badges1919 bronze badges

1 Answer
1

active

oldest

votes

9

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.

share|improve this answer

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That makes sense. But this means, that the "day" part of a Period can be anything between Integer.MIN_VALUE and Integer.MAX_VALUE making some quite interesting constellations possible even after normalization, right? (I.e., if your API accepts Period objects, you must essentially sanity-check them?)
  
  – D. Kovács
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yeah, they can take any value. I don't see why that needs to be sanitized. A period of 500000 days is a valid period, why wouldn't it be?
  
  – Max Vollmer
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Against malicious input... theoretical API for settings validity of max 2 years accepts a Period. Malicious input is 0 years, 1 month, 3650 days. You normalize and check year <= 2 and months accordingly. Yet, you set 10 years and a 1 month. (I know, it's a stretch, but you get the idea.)
  
  – D. Kovács
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I would check the date I get as a result from using the period, not the period itself. But I guess it really depends on what the API is actually doing.
  
  – Max Vollmer
  6 hours ago
  
  

  
  
  
  

add a comment | 

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9

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.

share|improve this answer

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That makes sense. But this means, that the "day" part of a Period can be anything between Integer.MIN_VALUE and Integer.MAX_VALUE making some quite interesting constellations possible even after normalization, right? (I.e., if your API accepts Period objects, you must essentially sanity-check them?)
  
  – D. Kovács
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yeah, they can take any value. I don't see why that needs to be sanitized. A period of 500000 days is a valid period, why wouldn't it be?
  
  – Max Vollmer
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Against malicious input... theoretical API for settings validity of max 2 years accepts a Period. Malicious input is 0 years, 1 month, 3650 days. You normalize and check year <= 2 and months accordingly. Yet, you set 10 years and a 1 month. (I know, it's a stretch, but you get the idea.)
  
  – D. Kovács
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I would check the date I get as a result from using the period, not the period itself. But I guess it really depends on what the API is actually doing.
  
  – Max Vollmer
  6 hours ago
  
  

  
  
  
  

add a comment | 

9

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.

share|improve this answer

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That makes sense. But this means, that the "day" part of a Period can be anything between Integer.MIN_VALUE and Integer.MAX_VALUE making some quite interesting constellations possible even after normalization, right? (I.e., if your API accepts Period objects, you must essentially sanity-check them?)
  
  – D. Kovács
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yeah, they can take any value. I don't see why that needs to be sanitized. A period of 500000 days is a valid period, why wouldn't it be?
  
  – Max Vollmer
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Against malicious input... theoretical API for settings validity of max 2 years accepts a Period. Malicious input is 0 years, 1 month, 3650 days. You normalize and check year <= 2 and months accordingly. Yet, you set 10 years and a 1 month. (I know, it's a stretch, but you get the idea.)
  
  – D. Kovács
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I would check the date I get as a result from using the period, not the period itself. But I guess it really depends on what the API is actually doing.
  
  – Max Vollmer
  6 hours ago
  
  

  
  
  
  

add a comment | 

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.

share|improve this answer

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

share|improve this answer

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges

answered 8 hours ago

Max VollmerMax Vollmer

6,37455 gold badges2121 silver badges3838 bronze badges