YYYY? yyyy!

Do you know what's the difference between the 'Y' and 'y' characters in the Java date pattern? In this article, we'll explore how an incorrect date format can cause an error. We'll also introduce our new V6122 diagnostic rule for Java that'll save you from sudden time travel.

class. Below is a small fragment of the table that describes how the date pattern interprets the alphabetic characters we're interested in:

A week year? Let me explain.

The week year is a year based on the week number of the year. What does it mean and why is it important?

In certain tasks, the sequence number of the week in the year is important. To determine the sequence number of the week, we need to decide which week to consider first. This is because the transition from one year to another often results in some weeks straddling both years. So, how can we tell which year this week belongs to? The ISO-8601 standard regulates this.

According to this standard, the first week of the year must meet the following conditions:

• the first day of the week is Monday;


• the minimum number of days of the year in a week is four.

From this, we can deduce a simple rule: a week is considered to be the first of the year if Thursday falls in January.

A real-life example can clarify this further.

Let's take the date from the example, which is 31.12.2024. Below is the calendar snippet for the week that includes our date (December 2024–January 2025):

This week is considered the first week of 2025 because it satisfies the above conditions (it has five January days). So, if we use the 'Y' specifier, we get the 2025 year.

As you can guess, the case with DateTimeFormatter is exactly the same.

It's important to note that we can travel not only to the future but also to the past.

Let's take another date, 01.01.2027.

Will the first week of 2027 include January 1? Again, let's consult the calendar.

Below is the calendar snippet for the week that includes our date (December 2026–January 2027):

As there are only three January days in this week (and it should be four in the first week according to the requirements), it counts as the last week of 2026. So, the date, when formatted using the 'Y' character, will show us 2026.

Here's the evidence. Take a look at the code:

public static void main(String[] args) {

    LocalDate date = LocalDate.of(2027, 1, 1);

    var formatter = DateTimeFormatter.ofPattern("dd-MM-YYYY");

    System.out.println(formatter.format(date));

}

The console displays the following:

01-01-2026

Okay, we've dissected the issue. We also found it interesting enough to add a dedicated diagnostic rule to PVS-Studio Java analyzer.

Errors in real projects

The diagnostic rule is written, so it's time to test it.

During the development of the analyzer, one of the testing stages involves running regression tests. We Usage of 'Y' (week year) pattern was detected: it was probably intended to use 'y' (year). SkeinParameters.java 246

Firstly, I took a look at GitHub. What if it really was a bug, the developers had already found it and committed a fix? That's exactly what happened. Here's a Usage of 'Y' (week year) pattern was detected: it was probably intended to use 'y' (year). RepositoryInfo.java 77

The code:

public class RepositoryInfo implements Serializable {

    ....

    protected static final SimpleDateFormat 

        outputDateFormat = new SimpleDateFormat("YYYY-MM-dd HH:mm Z"); 

    ....

}

Just as with the previous project, I rushed to the commits to see what was going on. First of all, it's worth noting that the field moved to its derived class, Repository, as a result of refactoring ( containing the fix. The 'Y' character in the date pattern has been replaced with 'y':

public abstract class Repository extends RepositoryInfo {

    ....

    protected static final SimpleDateFormat 

        OUTPUT_DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm Z");

    ....

}

So, it was an error here as well.

Conclusion

At PVS-Studio, we welcome any feedback from the community. After addressing the comment from a Habr user, we've enhanced the Java analyzer by adding some useful diagnostic rules. So, if you have any thoughts you'd like to share with us, we'd love to chat with you in the comment section of this article.

By the way, the diagnostic rule has been introduced in the October 7.33 release. So, if you want to try our analyzer, use this link.

That's all. Let's wrap things up here. Hopefully, the sudden trip a year forward (or backward) won't catch you off guard.