Why Using “YYYY‑MM‑dd” Breaks Your Java Dates on New Year's Day

Problem Overview

During a project a bug surfaced on New Year's Day when dates were formatted with the pattern YYYY-MM-dd. The resulting dates were off by one year, leading to failures in the application.

Below are screenshots showing the incorrect outputs:

Root Cause

In Java date‑time formatting, the pattern YYYY stands for the week‑based year , not the calendar year. A week is considered to start on Sunday and end on Saturday; if a week spans the turn of the year, the whole week is counted as belonging to the next year.

Consequently, using YYYY‑MM‑dd on dates near the year boundary (e.g., 2019‑12‑31) yields a year value of 2020, while the correct calendar year is 2019. The proper pattern for calendar dates is yyyy, which always reflects the actual year.

Recommendation

For everyday date formatting, always use yyyy‑MM‑dd. Reserve YYYY only for calculations that specifically require the week‑based year.

By paying attention to this subtle difference, developers can avoid similar bugs in the future.