[Bug] Default date converters silently normalize invalid calendar dates #1157

Fesod version

Current main at 5a9a16b.

Description

Default string-to-date conversion is lenient. Invalid calendar dates are silently changed into different valid dates instead of failing conversion.

Location

• fesod-sheet/src/main/java/org/apache/fesod/sheet/util/DateUtils.java:378-395
• fesod-sheet/src/main/java/org/apache/fesod/sheet/util/DateUtils.java:398-411

SimpleDateFormat keeps its default lenient behavior. DateTimeFormatter.ofPattern uses SMART resolver behavior unless made strict.

Reproduction

DateUtils.parseDate("2024-02-31");
// formats back as 2024-03-02

DateUtils.parseLocalDate("2024-02-31", null, Locale.US);
// returns 2024-02-29

A valid control value such as 2024-02-29 remains unchanged.

Expected behavior

Invalid calendar dates should raise a conversion error. Importing malformed spreadsheet text must not silently change the represented date.

Suggested fix

Disable lenient SimpleDateFormat parsing and use strict resolver semantics for java.time parsing. Account for yyyy versus uuuu when making existing patterns strict, and add converter-level regression tests for invalid month-end and leap-day values.

Related existing work

#1116/#1117 cover negative Excel serial dates. #1040 covers locale handling. Neither covers invalid string dates being normalized.

Are you willing to submit a PR?

Yes.