Bug with conversion Date to TO String

This is not a bug.

From the DateTimeFormatter documentation:, indicating the symbol, meaning and examples:

Y week-based-year year 1996; 96

So you're formatting the week-based-year, not the regular year. December 30th 2019 belongs to the first week of 2020, hence your output.

Use yyyy (year-of-era) or uuuu (year) instead of YYYY and you'll get 2019 instead.

Basically, YYYY should usually be used with w (week-of-week-based-year) and E (day-of-week).

Source: java - DateTimeFormatter adding a year to date after formatting - Stack Overflow