This post would have been more appropriate a week ago when 2018 was coming to a close but better late than never. This is a friendly reminder that when formatting dates in Java’s SimpleDateFormat class there is a subtle difference between YYYY and yyyy. They both represent a year but yyyy represents the calendar year while YYYY represents the year of the week. That’s a subtle difference that only causes problems around a year change so your code could have been running perfectly fine all year only to cause a problem in the new year.

An example illustrates this much better than words ever could.

package com.dangoldin.test;

import java.text.SimpleDateFormat;
import java.util.Date;

public class Test {

    public static void main(String[] args) {
        try {
            String[] dates = {"2018-12-01", "2018-12-31", "2019-01-01"};
            for (String date: dates) {
                SimpleDateFormat dt = new SimpleDateFormat("yyyy-MM-dd");
                Date d = dt.parse(date);

                SimpleDateFormat dtYYYY = new SimpleDateFormat("YYYY");
                SimpleDateFormat dtyyyy = new SimpleDateFormat("yyyy");

                System.out.println("For date " + date + " the YYYY year is " + dtYYYY.format(d) + " while for yyyy it's " + dtyyyy.format(d));
            }
        } catch (Exception e) {
            System.out.println("Failed with exception: " + e);
        }
    }
}

This gives you the following:

For date 2018-12-01 the YYYY year is 2018 while for yyyy it's 2018
For date 2018-12-31 the YYYY year is 2019 while for yyyy it's 2018
For date 2019-01-01 the YYYY year is 2019 while for yyyy it's 2019

The first and last make sense since the two year formats match. The middle one is the odd one out. The date starts as 2018-12-31 but YYYY gives you 2019 while yyyy gives you 2018. In general, you should almost always use yyyy so it’s a good tactic to add some form of linting or checking to make sure your code does not have any date formats referencing YYYY.

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