diff --git a/CHANGELOG.md b/CHANGELOG.md
index 57df162..bfab7cd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 #### Review fixes
 
+- Rejected invalid ISO calendar dates in time estimates instead of silently rolling them into another month.
 - Ignored empty relative file links wrapped in punctuation or spelled with dot path segments, instead of linking to the Todo document's directory.
 - Kept tag cleanup and hidden-tag labels working after an unmatched backtick, while preserving tags inside complete inline code spans.
 - Skipped false embedded-todo scan candidates when custom expressions match without a non-empty marker.
diff --git a/src/utils/time.ts b/src/utils/time.ts
index dc4d17f..1b8833b 100644
--- a/src/utils/time.ts
+++ b/src/utils/time.ts
@@ -272,6 +272,24 @@ const Time = {
 
       if (/^\s*\d+\s*$/.test(to)) return 0;
 
+      // Sugar normalizes impossible ISO dates (e.g. February 30) into another
+      // month; reject those explicitly rather than silently changing a deadline.
+      if (
+        /^\d{4}-\d{2}-\d{2}(?:[ T]\d{2}:\d{2}(?::\d{2})?)?$/.test(to) &&
+        !moment(
+          to,
+          [
+            'YYYY-MM-DD',
+            'YYYY-MM-DD HH:mm',
+            'YYYY-MM-DD HH:mm:ss',
+            'YYYY-MM-DDTHH:mm',
+            'YYYY-MM-DDTHH:mm:ss',
+          ],
+          true
+        ).isValid()
+      )
+        return 0;
+
       // Parse duration strings before calendar expressions so they stay relative to `from`.
       try {
         const milliseconds = toTime(to).milliseconds();
diff --git a/test/time.spec.ts b/test/time.spec.ts
index 36dda38..251b82d 100644
--- a/test/time.spec.ts
+++ b/test/time.spec.ts
@@ -74,6 +74,14 @@ describe('Time utilities', () => {
     expect(Time.durationSeconds('2020-01-02', from)).to.equal(24 * 3600);
   });
 
+  it('rejects invalid ISO calendar dates rather than rolling them into another month', () => {
+    const from = new Date(2020, 0, 1);
+
+    expect(Time.durationSeconds('2020-02-30', from)).to.equal(0);
+    expect(Time.diffSeconds('2021-02-29 09:00', from)).to.equal(0);
+    expect(Time.durationSeconds('2020-02-29', from)).to.equal(59 * 24 * 3600);
+  });
+
   it('supports flat man-hours formatting', () => {
     const from = new Date('2020-01-01T00:00:00Z');
     const to = new Date(from.getTime() + (25 * 3600 + 15 * 60) * 1000);