diff --git a/examples/add-mo-out-of-range.ilo b/examples/add-mo-out-of-range.ilo
new file mode 100644
index 00000000..91ee46c6
--- /dev/null
+++ b/examples/add-mo-out-of-range.ilo
@@ -0,0 +1,12 @@
+-- add-mo handles large but representable month offsets without
+-- panicking or wrapping. Pre-fix, the internal arithmetic used i32
+-- and overflowed for big offsets (panic in debug, silent wrap in
+-- release). Post-fix it widens to i64 and either returns a sensible
+-- epoch or surfaces a clean ILO-R009 "result out of calendar range".
+-- run: thousand-years
+-- out: 31556995200
+
+-- 1000 years forward of 1970-01-01 = 12000 calendar months.
+-- 2970-01-01 00:00 UTC = 31556995200.
+thousand-years > n
+  add-mo 0 12000
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index bb45f72b..74f494b1 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -2006,12 +2006,20 @@ fn add_mo_impl(epoch_arg: &Value, months_arg: &Value) -> Result<Value> {
     };
     let date = dt.date_naive();
     fn add_months_snap(date: NaiveDate, months: i32) -> Option<NaiveDate> {
-        let total = date.year() * 12 + (date.month() as i32 - 1) + months;
-        let y = total.div_euclid(12);
+        // Compute total months since year-0 in i64 so adding i32::MAX (or
+        // i32::MIN) months to any chrono-representable year cannot wrap.
+        // Pre-fix this was i32 arithmetic: `date.year() * 12 + months`
+        // overflowed at i32::MAX months, panicking in debug and silently
+        // wrapping to a valid date in release. The widened path now either
+        // produces a valid NaiveDate or returns None, which the caller
+        // surfaces as a clean ILO-R009 "result out of calendar range".
+        let total: i64 = (date.year() as i64) * 12 + (date.month() as i64 - 1) + (months as i64);
+        let y_i64 = total.div_euclid(12);
         let m = (total.rem_euclid(12) + 1) as u32;
+        let y: i32 = i32::try_from(y_i64).ok()?;
         let max_day = {
             let next = if m == 12 {
-                NaiveDate::from_ymd_opt(y + 1, 1, 1)
+                NaiveDate::from_ymd_opt(y.checked_add(1)?, 1, 1)
             } else {
                 NaiveDate::from_ymd_opt(y, m + 1, 1)
             };
diff --git a/tests/regression_calendar_arithmetic.rs b/tests/regression_calendar_arithmetic.rs
index 41f7c786..9cd5a7f0 100644
--- a/tests/regression_calendar_arithmetic.rs
+++ b/tests/regression_calendar_arithmetic.rs
@@ -405,6 +405,9 @@ fn add_mo_epoch_out_of_range() {
 fn add_mo_result_out_of_calendar_range() {
     // Max i32 months from epoch 0 pushes far past chrono's max year (262143).
     // Exercises the `None` arm of `match add_months_snap(date, months)`.
+    // Regression: the year-as-months arithmetic used to be i32 and silently
+    // wrapped in release / panicked in debug at i32::MAX; it now widens to
+    // i64 and surfaces a clean ILO-R009.
     check_err(
         "f>n;add-mo 0 2147483647",
         &["f"],
@@ -412,6 +415,31 @@ fn add_mo_result_out_of_calendar_range() {
     );
 }
 
+#[test]
+fn add_mo_result_out_of_calendar_range_negative() {
+    // Symmetric negative case: i32::MIN months from epoch 0 pushes far before
+    // chrono's min year. Guards against an asymmetric overflow fix that only
+    // handles the positive side.
+    check_err(
+        "f>n;add-mo 0 -2147483648",
+        &["f"],
+        "add-mo: result out of calendar range",
+    );
+}
+
+#[test]
+fn add_mo_large_but_valid_offset() {
+    // 1000 years forward of 1970-01-01 lands on 2970-01-01, well within
+    // chrono's representable range. Guards against the overflow fix being
+    // too aggressive and rejecting plausible long offsets.
+    // 2970-01-01 00:00 UTC = 31556995200
+    check_num(
+        "f dt:n n:n>n;add-mo dt n",
+        &["f", "0", "12000"],
+        31556995200.0,
+    );
+}
+
 #[test]
 fn last_dom_epoch_out_of_range() {
     check_err(