What's the Difference Between 'Y' and 'y' in Java's SimpleDateFormat?

SimpleDateFormat Formats: 'Y' vs 'y'

The Java SimpleDateFormat class allows you to format dates and times using various patterns. Two patterns that may seem similar, 'Y' and 'y', actually have subtle differences that can lead to unexpected results.

'Y' - Week Year

The 'Y' pattern formats the week year. A week year is aligned with a WEEK_OF_YEAR cycle, meaning all weeks within that cycle have the same week year value. However, the first and last days of a week year may have different calendar years.

For example, if the first day of the week is Monday and the minimum number of days in the first week is set to 4 (ISO 8601 standard), then week 1 of 2018 starts on December 29, 2017, and ends on January 4, 2018. In this case, the week year for the last three days of 2017 is considered to be 2018.

'y' - Year

On the other hand, the 'y' pattern formats the calendar year. It represents the year number within the Gregorian calendar, regardless of the week cycle.

Sample Comparison

System.out.println(new SimpleDateFormat("Y").format(new Date())); // prints 2012
System.out.println(new SimpleDateFormat("y").format(new Date())); // prints 2011

In this example, 'Y' returns 2012 because the current date is within week 1 of 2018, and the week year for the last three days of 2017 is also considered to be 2018. However, 'y' returns 2011 because it only considers the calendar year, which is 2017 for the beginning of week 1.

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