fix(fill): respect workbook date system in monthly autofill

crates/engine/src/fill.rs

@@ -165,6 +165,7 @@ enum Pattern {
         last: f64,
         first: f64,
         months: i64,
+        sys: gridcraft_core::DateSystem,
     },
 }
 
@@ -199,7 +200,7 @@ impl Pattern {
                     && (a.month != b.month || a.year != b.year)
                 {
                     let months = (b.year as i64 - a.year as i64) * 12 + b.month as i64 - a.month as i64;
-                    return Pattern::Months { last, first, months };
+                    return Pattern::Months { last, first, months, sys };
                 }
             }
             // Least-squares step for 2+ values (Excel uses a linear trend).
@@ -297,9 +298,9 @@ impl Pattern {
                     s
                 }))
             }
-            Pattern::Months { last, first, months } => {
+            Pattern::Months { last, first, months, sys } => {
                 let base = if forward { *last } else { *first };
-                let s = add_months_clamped(gridcraft_core::DateSystem::D1900, base, months.checked_mul(k * sign)?)?;
+                let s = add_months_clamped(*sys, base, months.checked_mul(k * sign)?)?;
                 Some(Value::number(s))
             }
         }

crates/engine/src/tests.rs

@@ -2897,6 +2897,33 @@ fn fill_series_year_clamps_leap_day() {
     assert_eq!(v(&s, "A5"), ymd(2032, 2, 29));
 }
 
+#[test]
+fn autofill_month_pattern_uses_workbook_date_system() {
+    use std::sync::Arc;
+
+    use gridcraft_core::date::serial_from_ymd;
+    use gridcraft_core::{DateSystem, RangeRef};
+
+    for sys in [DateSystem::D1900, DateSystem::D1904] {
+        for (source, target, seeds, expected) in [
+            ("A1:A2", "A1:A6", ["2026-01-31", "2026-03-31"], ["2026-01-31", "2026-03-31", "2026-05-31", "2026-07-31", "2026-09-30", "2026-11-30"]),
+            ("A5:A6", "A1:A6", ["2026-09-30", "2026-11-30"], ["2026-01-30", "2026-03-30", "2026-05-30", "2026-07-30", "2026-09-30", "2026-11-30"]),
+        ] {
+            let mut s = s();
+            Arc::make_mut(&mut s.doc_mut().unwrap().wb).date_system = sys;
+            let start = RangeRef::parse(source).unwrap().start.a1();
+            s.execute("range.setValues", json!({"range": start, "values": [[seeds[0]], [seeds[1]]]})).unwrap();
+            s.execute("edit.autoFill", json!({"source": source, "target": target})).unwrap();
+            for (row, date) in expected.iter().enumerate() {
+                let parts: Vec<i64> = date.split('-').map(|p| p.parse().unwrap()).collect();
+                let expected = Value::Number(serial_from_ymd(sys, parts[0], parts[1], parts[2]).unwrap());
+                let cell = CellRef::new(row as u32, 0).a1();
+                assert_eq!(v(&s, &cell), expected, "{sys:?} {source} -> {target}, {cell}: {date}");
+            }
+        }
+    }
+}
+
 #[test]
 fn autofill_month_pattern_clamps_to_month_end() {
     let mut s = s();

docs/ui-parity.md

@@ -1,6 +1,6 @@
 # UI parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-13 · **Change:** minor (horizontal scrollbar in the sheet tab bar, #840) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-13 · **Change:** minor (AutoFill month patterns respect the workbook's date system) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Interaction, controls and feel, against Excel. Part of [target-app-parity.md](target-app-parity.md).
 **UI/UX fidelity: ~55% ready, 50–80 h** (estimated). Ribbon command coverage is measured
@@ -45,7 +45,7 @@ this logic, with one undo step.
 |---|---|
 | Click/drag/Shift/Ctrl selection, multi-area selection | rectangular drag works in one direction only in some cases (#28) |
 | Selection outline glides to a new selection (Excel: "Provide feedback with animation") | done (10-11): 0.12 s, decelerating, with the fill and fill handle; View ▸ Animations (`view.animations`) turns it off. The timing is an estimate, not yet measured against Excel; the system's reduced-motion setting isn't followed yet |
-| Fill handle: drag to fill series, double-click to fill down, right-drag menu | fill done (Ctrl+drag swaps copy and series); right-drag menu missing |
+| Fill handle: drag to fill series, double-click to fill down, right-drag menu | fill done (Ctrl+drag swaps copy and series); month patterns use the workbook's 1900 or 1904 date system; right-drag menu missing |
 | Drag-move and drag-copy cells (border drag, Ctrl/Option, Shift to insert) | done (M3) |
 | In-cell editing, Enter/Tab movement, Alt/Option+Enter line breaks | done |
 | Point mode (click/drag/arrow references), reference colouring, F4 | done |
@@ -161,6 +161,7 @@ scrollbar bugs 2–4 · pixel/feel pass against observed Excel 7–12.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-13 | minor | AutoFill month patterns retain the workbook's date system, preventing shifted dates in 1904 workbooks when filling forward or backward |
 | 2026-10-12 | minor | The File view's recent list tells when each workbook was last changed, read off the UI thread |
 | 2026-10-13 | minor | Horizontal scrollbar in the sheet tab bar: drag, track click pages, frozen columns excluded from the page (#840) |
 | 2026-10-12 | minor | Ctrl+wheel zoom steps along a ladder of percentages about 20% apart that never skips 100%, gliding to each step with animations on, so a notch out undoes a notch in (it drifted: 100 → 122 → 101); Excel steps 15 points, see decisions.md |