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datetimeformatter: DateTimeFormatter cheat sheet module (pattern letters incl. the four hour letters, yyyy vs YYYY week-year bug and WeekFields, en_IN locales and the U+202F AM/PM trap with parseLenient fix, SMART/STRICT/LENIENT parsing, ISO constants, SimpleDateFormat thread safety), 18 contract tests

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Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b3fvyFpxThdRxcozLx
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Claude
2026-11-14 01:24:52 +05:30
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datetimeformatter/output/02-week-year-bug.txt
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== one date, two letters ==
date = 2026-12-29 (TUESDAY)
dd-MM-yyyy = 29-12-2026
dd-MM-YYYY = 29-12-2027
== the same YYYY pattern, four locales, three dates ==
date en_US en_GB fr_FR en_IN
2026-12-28 2027 2026 2026 2027
2026-12-29 2027 2026 2026 2027
2027-01-01 2027 2026 2026 2027
== why: each locale defines what a week is (WeekFields) ==
en_US WeekFields[SUNDAY,1] week-based-year of 2026-12-29 = 2027, week number = 1
en_GB WeekFields[MONDAY,4] week-based-year of 2026-12-29 = 2026, week number = 53
fr_FR WeekFields[MONDAY,4] week-based-year of 2026-12-29 = 2026, week number = 53
en_IN WeekFields[SUNDAY,1] week-based-year of 2026-12-29 = 2027, week number = 1
== how often is YYYY wrong? every day of 2020-2030, compared with yyyy ==
en_US 36 wrong days out of 4018
en_GB 19 wrong days out of 4018
fr_FR 19 wrong days out of 4018
en_IN 36 wrong days out of 4018
== the days that are wrong in the US locale ==
2024/2025: 2024-12-29->2025, 2024-12-30->2025, 2024-12-31->2025
2025/2026: 2025-12-28->2026, 2025-12-29->2026, 2025-12-30->2026, 2025-12-31->2026
2026/2027: 2026-12-27->2027, 2026-12-28->2027, 2026-12-29->2027, 2026-12-30->2027, 2026-12-31->2027
2027/2028: 2027-12-26->2028, 2027-12-27->2028, 2027-12-28->2028, 2027-12-29->2028, 2027-12-30->2028, 2027-12-31->2028
2028/2029: 2028-12-31->2029
== yyyy vs uuuu: same for modern dates, different for the era ==
date = -0043-03-15
yyyy = 0044
uuuu = -0043
yyyy G = 0044 BC