diff --git a/pylib/tests/test_schedv3.py b/pylib/tests/test_schedv3.py
index f106e7725..0deff7bf9 100644
--- a/pylib/tests/test_schedv3.py
+++ b/pylib/tests/test_schedv3.py
@@ -537,7 +537,7 @@ def test_nextIvl():
     assert ni(c, 3) == 21600000
     # (* 100 2.5 1.3 86400)28080000.0
     assert ni(c, 4) == 28080000
-    assert without_unicode_isolation(col.sched.nextIvlStr(c, 4)) == "10.8mo"
+    assert without_unicode_isolation(col.sched.nextIvlStr(c, 4)) == "10.7mo"
 
 
 def test_bury():
diff --git a/rslib/src/scheduler/timespan.rs b/rslib/src/scheduler/timespan.rs
index f98c1ce69..c779d33bc 100644
--- a/rslib/src/scheduler/timespan.rs
+++ b/rslib/src/scheduler/timespan.rs
@@ -57,7 +57,7 @@ const SECOND: f32 = 1.0;
 const MINUTE: f32 = 60.0 * SECOND;
 const HOUR: f32 = 60.0 * MINUTE;
 const DAY: f32 = 24.0 * HOUR;
-const MONTH: f32 = 30.0 * DAY;
+const MONTH: f32 = 30.417 * DAY; // 365/12 ≈ 30.417
 const YEAR: f32 = 365.0 * DAY;
 
 #[derive(Clone, Copy)]
@@ -188,6 +188,10 @@ mod test {
         assert_eq!(time_span(30.3, &tr, true), "30.3 seconds");
         assert_eq!(time_span(90.0, &tr, false), "1.5 minutes");
         assert_eq!(time_span(45.0 * 86_400.0, &tr, false), "1.5 months");
+        assert_eq!(time_span(364.0 * 86_400.0, &tr, false), "12 months");
+        assert_eq!(time_span(364.0 * 86_400.0, &tr, true), "11.97 months");
+        assert_eq!(time_span(365.0 * 86_400.0, &tr, false), "1 year");
+        assert_eq!(time_span(365.0 * 86_400.0, &tr, true), "1 year");
         assert_eq!(time_span(365.0 * 86_400.0 * 1.5, &tr, false), "1.5 years");
     }
 }
diff --git a/ts/lib/tslib/time.test.ts b/ts/lib/tslib/time.test.ts
index f8bf4eddf..801550f9e 100644
--- a/ts/lib/tslib/time.test.ts
+++ b/ts/lib/tslib/time.test.ts
@@ -12,7 +12,7 @@ test("natural unit", () => {
     expect(naturalUnit(60 * 60 - 1)).toBe(TimespanUnit.Minutes);
     expect(naturalUnit(60 * 60)).toBe(TimespanUnit.Hours);
     expect(naturalUnit(60 * 60 * 24)).toBe(TimespanUnit.Days);
-    expect(naturalUnit(60 * 60 * 24 * 30)).toBe(TimespanUnit.Months);
+    expect(naturalUnit(60 * 60 * 24 * 31)).toBe(TimespanUnit.Months);
 });
 
 test("natural whole unit", () => {
diff --git a/ts/lib/tslib/time.ts b/ts/lib/tslib/time.ts
index 6cb5df5a1..a517e8a39 100644
--- a/ts/lib/tslib/time.ts
+++ b/ts/lib/tslib/time.ts
@@ -7,8 +7,8 @@ export const SECOND = 1.0;
 export const MINUTE = 60.0 * SECOND;
 export const HOUR = 60.0 * MINUTE;
 export const DAY = 24.0 * HOUR;
-export const MONTH = 30.0 * DAY;
-export const YEAR = 12.0 * MONTH;
+export const MONTH = 30.417 * DAY; // 365/12 ≈ 30.417
+export const YEAR = 365.0 * DAY;
 
 export enum TimespanUnit {
     Seconds,