Howard Hinnant civil-date algorithms hand-written 7 times across 3 crates #6997

Describe the bug

Problem

Location: native/spark-expr/src/kernels/temporal.rs:402
Symbol: civil_from_days / days_from_civil

Grep finds days_from_civil and civil_from_days as separate private copies in: (1) native/spark-expr/src/kernels/temporal.rs:407 (civil_from_days) and :425 (days_from_civil); (2) native/spark-expr/src/conversion_funcs/string.rs:1300 (days_from_civil), plus a test copy at :3440; (3) native/core/src/parquet/datetime_rebase.rs:162 (days_from_civil); (4) native/spark-expr/src/iceberg_funcs/temporal.rs:132 (civil_from_days, i32 in, returns (year, month0)); (5) native/core/src/execution/operators/iceberg_partition_path.rs:278 (civil_from_days, i64 in, (i64,u32,u32) out). The copies have already drifted in how they do floor division: string.rs uses if y >= 0 { y / 400 } else { (y - 399) / 400 }, iceberg_partition_path.rs uses a manual shifted - 146_096 floor plus a cast to u64, and the others use div_euclid. Month indexing also differs ((m + 9) % 12 vs. if/else). Each copy is a separately maintained implementation of the same correctness-critical calendar math that Spark compatibility depends on.

Suggested fix:

Keep the div_euclid i64 versions from kernels/temporal.rs as pub fn days_from_civil(y, m, d) -> i64 and pub fn civil_from_days(days) -> (i64, i64, i64) in a crate that both spark-expr and core depend on (for example native/common, or a calendar module in spark-expr). Replace the other copies with thin adapters, such as (year as i32, month - 1) for Iceberg, and add one shared property test that checks the pair round-trips across the full i32 epoch-day range.

Code context

// Integer proleptic Gregorian calendar conversion, adapted from DataFusion 55.1.0's
// datetime/date_trunc.rs (Howard Hinnant's algorithms):
fn civil_from_days(days: i64) -> (i64, i64, i64) {
    let z = days + 719_468;
    let era = z.div_euclid(146_097);
    ...
}

fn days_from_civil(year: i64, month: i64, day: i64) -> i64 {
    let year = year - i64::from(month <= 2);
    let era = year.div_euclid(400);
    ...
    era * 146_097 + day_of_era - 719_468
}
// other copies: conversion_funcs/string.rs:1300, core/parquet/datetime_rebase.rs:162,
// iceberg_funcs/temporal.rs:132, core/.../iceberg_partition_path.rs:278

Steps to reproduce

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Expected behavior

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Additional context

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Willingness to contribute

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