diff --git a/extensions/logbook/src/analyze.test.ts b/extensions/logbook/src/analyze.test.ts
index 8be51d77e71b..c621e46a145e 100644
--- a/extensions/logbook/src/analyze.test.ts
+++ b/extensions/logbook/src/analyze.test.ts
@@ -31,6 +31,8 @@ describe("clockToMs", () => {
 
   it("rejects malformed input", () => {
     expect(clockToMs(DAY, "25:00:00")).toBeNull();
+    expect(clockToMs(DAY, "13:05 pm")).toBeNull();
+    expect(clockToMs(DAY, "00:05 am")).toBeNull();
     expect(clockToMs(DAY, "half past nine")).toBeNull();
     expect(clockToMs("not-a-day", "10:00:00")).toBeNull();
   });
@@ -160,7 +162,7 @@ describe("parseCardsJson", () => {
 
   it("reports actionable errors for the correction round-trip", () => {
     const result = parseCardsJson({
-      raw: JSON.stringify([card({ startTime: "later that day" })]),
+      raw: JSON.stringify([card({ startTime: "13:05 pm" })]),
       day: DAY,
       windowStartMs,
       windowEndMs,
diff --git a/extensions/logbook/src/analyze.ts b/extensions/logbook/src/analyze.ts
index 0ac0c9a27095..6b04d43937b3 100644
--- a/extensions/logbook/src/analyze.ts
+++ b/extensions/logbook/src/analyze.ts
@@ -24,6 +24,9 @@ export function clockToMs(day: string, clock: string): number | null {
   const minutes = Number(match[2]);
   const seconds = Number(match[3] ?? "0");
   const meridiem = match[4]?.toLowerCase();
+  if (meridiem && (hours < 1 || hours > 12)) {
+    return null;
+  }
   if (meridiem === "pm" && hours < 12) {
     hours += 12;
   }