From 960c64c3e44de2c0ae99012d544b52f3278fed37 Mon Sep 17 00:00:00 2001
From: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>
Date: Sun, 25 Oct 2026 02:19:51 +0000
Subject: [PATCH] Return None from datetime_from_serial when rounding carries
 past 9999

Found while testing crates/core/src/date.rs:168-177

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 crates/core/src/date.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/crates/core/src/date.rs b/crates/core/src/date.rs
index b42c068a0..8e68695f1 100644
--- a/crates/core/src/date.rs
+++ b/crates/core/src/date.rs
@@ -174,6 +174,9 @@ pub fn datetime_from_serial(sys: DateSystem, serial: f64) -> Option<DateTime> {
     if frac_ms >= 86_400_000 {
         frac_ms -= 86_400_000;
         day += 1;
+        if day > max_serial(sys) {
+            return None;
+        }
     }
     let (y, m, d, weekday) = match sys {
         DateSystem::D1900 => {
@@ -221,6 +224,14 @@ pub const WEEKDAYS: [&str; 7] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Th
 mod tests {
     use super::*;
 
+    #[test]
+    fn serial_rounding_into_next_day_past_max_is_none() {
+        for sys in [DateSystem::D1900, DateSystem::D1904] {
+            let serial = max_serial(sys) as f64 + 0.9999999995;
+            assert_eq!(datetime_from_serial(sys, serial), None);
+        }
+    }
+
     #[test]
     fn leap_and_month_length_outside_chrono_range() {
         // In range, the ordinary Gregorian rule holds.