A real‑world bug caused by incorrect year‑week calculation is explained, then the ISO‑8601 week‑based year definition and Java's LocalDate/WeekFields APIs are introduced to reliably compute the correct year and week for any date.
The article explains why using the pattern 'YYYY‑MM‑dd' with Java's SimpleDateFormat can produce an incorrect year for dates near the end of the year, demonstrates the issue with a unit test comparing it to the correct 'yyyy‑MM‑dd' pattern, and reveals that the problem stems from the week‑based year semantics of 'Y'.
This article demonstrates how the week‑based year pattern "YYYY" in Java's SimpleDateFormat can incorrectly roll over the year for dates near year‑end, provides a reproducible unit test with code and output, and explains the underlying reason behind the discrepancy.
