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
Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b3fvyFpxThdRxcozLx
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Claude
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moment = 2026-11-14T01:17:05.123456789+05:30[Asia/Kolkata] (a Saturday, day 318 of the year)
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yyyy -> 2026
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yy -> 26
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uuuu -> 2026
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M -> 11
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MM -> 11
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MMM -> Nov
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MMMM -> November
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MMMMM -> N
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d -> 14
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dd -> 14
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D -> 318
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DDD -> 318
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E -> Sat
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EEEE -> Saturday
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EEEEE -> S
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e -> 7
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ee -> 07
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H -> 1
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HH -> 01
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h -> 1
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hh -> 01
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k -> 1
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K -> 1
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a -> AM
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B -> in the morning
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BBBB -> in the morning
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m -> 17
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mm -> 17
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s -> 5
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ss -> 05
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S -> 1
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SSS -> 123
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SSSSSS -> 123456
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n -> 123456789
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A -> 4625123
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G -> AD
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Q -> 4
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QQQ -> Q4
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w -> 46
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W -> 2
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z -> IST
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zzzz -> India Standard Time
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VV -> Asia/Kolkata
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O -> GMT+5:30
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OOOO -> GMT+05:30
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X -> +0530
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XXX -> +05:30
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x -> +0530
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xxx -> +05:30
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Z -> +0530
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ZZZZ -> GMT+05:30
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ZZZZZ -> +05:30
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== the four hour letters, at midnight, 01:00, noon and 13:00 ==
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time H h k K
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00:17 0 12 24 0
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01:17 1 1 1 1
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12:17 12 12 12 0
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13:17 13 1 13 1
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== one date, two letters ==
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date = 2026-12-29 (TUESDAY)
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dd-MM-yyyy = 29-12-2026
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dd-MM-YYYY = 29-12-2027
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== the same YYYY pattern, four locales, three dates ==
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date en_US en_GB fr_FR en_IN
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2026-12-28 2027 2026 2026 2027
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2026-12-29 2027 2026 2026 2027
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2027-01-01 2027 2026 2026 2027
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== why: each locale defines what a week is (WeekFields) ==
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en_US WeekFields[SUNDAY,1] week-based-year of 2026-12-29 = 2027, week number = 1
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en_GB WeekFields[MONDAY,4] week-based-year of 2026-12-29 = 2026, week number = 53
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fr_FR WeekFields[MONDAY,4] week-based-year of 2026-12-29 = 2026, week number = 53
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en_IN WeekFields[SUNDAY,1] week-based-year of 2026-12-29 = 2027, week number = 1
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== how often is YYYY wrong? every day of 2020-2030, compared with yyyy ==
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en_US 36 wrong days out of 4018
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en_GB 19 wrong days out of 4018
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fr_FR 19 wrong days out of 4018
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en_IN 36 wrong days out of 4018
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== the days that are wrong in the US locale ==
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2024/2025: 2024-12-29->2025, 2024-12-30->2025, 2024-12-31->2025
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2025/2026: 2025-12-28->2026, 2025-12-29->2026, 2025-12-30->2026, 2025-12-31->2026
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2026/2027: 2026-12-27->2027, 2026-12-28->2027, 2026-12-29->2027, 2026-12-30->2027, 2026-12-31->2027
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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
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2028/2029: 2028-12-31->2029
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== yyyy vs uuuu: same for modern dates, different for the era ==
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date = -0043-03-15
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yyyy = 0044
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uuuu = -0043
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yyyy G = 0044 BC
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== the same pattern in four locales ==
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en_US Tuesday, 1 September 2026
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en_IN Tuesday, 1 September 2026
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hi_IN मंगलवार, 1 सितंबर 2026
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mr_IN मंगळवार, 1 सप्टेंबर 2026
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== abbreviated month: not what every Indian or British reader expects ==
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en_US 1 Sep 2026
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en_GB 1 Sept 2026
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en_IN 1 Sept 2026
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== localized styles for en_IN (the pattern is chosen by the locale, not by you) ==
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MEDIUM 1 Sept 2026
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SHORT 01/09/26
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FULL Tuesday, 1 September 2026
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== AM/PM text per locale, with the hex of every character ==
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en_US [03:30 PM] 30 33 3A 33 30 20 50 4D
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en_IN [03:30 pm] 30 33 3A 33 30 20 70 6D
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== ofLocalizedTime(SHORT): the space before PM is not a space ==
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en_US 33 3A 33 30 202F 50 4D
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en_IN 33 3A 33 30 202F 70 6D
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== parsing it back with a plain space ==
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DateTimeParseException: Text '3:30 PM' could not be parsed at index 4
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15:30
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== the other direction: text that CONTAINS U+202F, read by a pattern with a plain space ==
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DateTimeParseException: Text '3:30_PM' could not be parsed at index 4
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== parseLenient() accepts either kind of space, in both directions ==
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localized SHORT, input with plain space : 15:30
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pattern h:mm a, input with U+202F : 15:30
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== 31 February under three resolver styles (pattern dd-MM-uuuu) ==
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STRICT DateTimeParseException: Text '31-02-2026' could not be parsed: Invalid date 'FEBRUARY 31'
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SMART 2026-02-28
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LENIENT 2026-03-03
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== 30 February and 32 January ==
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SMART 30-02-2026 2026-02-28
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SMART 32-01-2026 DateTimeParseException: Text '32-01-2026' could not be parsed: Invalid value for DayOfMonth (valid values 1 - 28/31): 32
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== STRICT with yyyy: year-of-era needs an era, so it cannot resolve ==
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yyyy DateTimeParseException: Text '14-11-2026' could not be parsed: Unable to obtain LocalDate from TemporalAccessor: {MonthOfYear=11, DayOfMonth=14, YearOfEra=2026},ISO of type java.time.format.Parsed
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uuuu 2026-11-14
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== how many digits? MM wants two, M accepts one or two ==
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yyyy-MM-dd 2026-1-5 DateTimeParseException: Text '2026-1-5' could not be parsed at index 5
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yyyy-M-d 2026-1-5 2026-01-05
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yyyy-M-d 2026-01-05 2026-01-05
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== two-digit years are read as 2000-2099 ==
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01-01-30 -> 2030-01-01
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01-01-99 -> 2099-01-01
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== month names are case-sensitive unless you say otherwise ==
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14 Nov 2026 2026-11-14
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14 NOV 2026 DateTimeParseException: Text '14 NOV 2026' could not be parsed at index 3
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14 NOV 2026 (parseCaseInsensitive) 2026-11-14
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== the old SimpleDateFormat, same 31-02-2026 ==
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lenient (default): Tue Mar 03 00:00:00 IST 2026
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setLenient(false): ParseException: Unparseable date: "31-02-2026"
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moment = 2026-12-29T01:17:05.123+05:30[Asia/Kolkata]
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ISO_LOCAL_DATE 2026-12-29
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ISO_LOCAL_TIME 01:17:05.123
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ISO_LOCAL_DATE_TIME 2026-12-29T01:17:05.123
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ISO_OFFSET_DATE_TIME 2026-12-29T01:17:05.123+05:30
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ISO_ZONED_DATE_TIME 2026-12-29T01:17:05.123+05:30[Asia/Kolkata]
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ISO_INSTANT 2026-12-28T19:47:05.123Z
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ISO_DATE_TIME 2026-12-29T01:17:05.123+05:30[Asia/Kolkata]
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ISO_ORDINAL_DATE 2026-363+05:30
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ISO_WEEK_DATE 2026-W53-2+05:30
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BASIC_ISO_DATE 20261229+0530
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RFC_1123_DATE_TIME Tue, 29 Dec 2026 01:17:05 +0530
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== ISO_INSTANT on a ZonedDateTime converts to UTC ==
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2026-12-28T19:47:05.123Z
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== toString() is already ISO for the java.time types ==
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2026-12-29 | 01:17:05.123 | 2026-12-29T01:17:05.123+05:30
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hour pattern on a LocalDate UnsupportedTemporalTypeException: Unsupported field: HourOfDay
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zone pattern on a LocalDateTime DateTimeException: Unable to extract ZoneId from temporal 2026-11-14T01:17
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unknown pattern letter 'b' IllegalArgumentException: Unknown pattern letter: b
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unterminated quote IllegalArgumentException: Pattern ends with an incomplete string literal: uuuu-MM-dd'T
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the T in an ISO pattern, unquoted IllegalArgumentException: Unknown pattern letter: T
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the T in an ISO pattern, quoted OK: 2026-11-14T01:17
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text too short DateTimeParseException: Text '2026-11' could not be parsed at index 7
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text too long DateTimeParseException: Text '2026-11-14 extra' could not be parsed, unparsed text found at index 10
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date-only text into a LocalDateTime DateTimeParseException: Text '2026-11-14' could not be parsed: Unable to obtain LocalDateTime from TemporalAccessor: {},ISO resolved to 2026-11-14 of type java.time.format.Parsed
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month name in the wrong language DateTimeParseException: Text '14 novembre 2026' could not be parsed at index 3
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same text, French locale OK: 2026-11-14
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threads=8, parses per thread=20000
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SimpleDateFormat (shared) : exceptions=461, wrong results=3824
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DateTimeFormatter (shared): exceptions=0, wrong results=0
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[INFO] Tests run: 18, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.918 s -- in com.ankurm.datetimeformatter.DateTimeFormatterTest
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[INFO] Tests run: 18, Failures: 0, Errors: 0, Skipped: 0
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[INFO] BUILD SUCCESS
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