diff --git a/packages/protocol/src/oidb-services/contacts/fetch-user-profile.ts b/packages/protocol/src/oidb-services/contacts/fetch-user-profile.ts
index 37bcd980..3b07e07f 100644
--- a/packages/protocol/src/oidb-services/contacts/fetch-user-profile.ts
+++ b/packages/protocol/src/oidb-services/contacts/fetch-user-profile.ts
@@ -94,7 +94,7 @@ export function applyProfileDetails(info: UserProfileInfo, numMap: Map<number, n
 export function applyBirthday(info: UserProfileInfo, bytesMap: Map<number, Uint8Array>): void {
     const raw = bytesMap.get(20031);
 
-    if (!raw || raw.length < 4) {
+    if (raw?.length !== 4) {
         return;
     }
 
@@ -103,7 +103,11 @@ export function applyBirthday(info: UserProfileInfo, bytesMap: Map<number, Uint8
     const month = bytes.readUInt8(2);
     const day = bytes.readUInt8(3);
 
-    if (month < 1 || month > 12 || day < 1 || day > 31) {
+    // A hidden year can still represent February 29.
+    const date = new Date(0);
+    date.setUTCFullYear(year || 2000, month - 1, day);
+
+    if (date.getUTCMonth() !== month - 1 || date.getUTCDate() !== day) {
         return;
     }
 
diff --git a/packages/protocol/tests/oidb-services/contacts/fetch-user-profile.test.ts b/packages/protocol/tests/oidb-services/contacts/fetch-user-profile.test.ts
index 0a313cb3..73ef86a5 100644
--- a/packages/protocol/tests/oidb-services/contacts/fetch-user-profile.test.ts
+++ b/packages/protocol/tests/oidb-services/contacts/fetch-user-profile.test.ts
@@ -148,17 +148,22 @@ describe('FetchUserProfile namespace', () => {
             },
         );
 
-        it('omits only the birth year when the year is hidden', async () => {
+        it.each([
+            [12, 31],
+            [2, 29],
+        ])('keeps a hidden-year birthday %i/%i', async (month, day) => {
             const sender = makeSender({
                 body: {
                     uin: 10001,
                     properties: {
-                        bytesProperties: [{ code: 20031, value: new Uint8Array([0, 0, 12, 31]) }],
+                        bytesProperties: [
+                            { code: 20031, value: new Uint8Array([0, 0, month, day]) },
+                        ],
                     },
                 },
             });
             const profile = await FetchUserProfile.invoke(sender, { uin: 10001 });
-            expect(profile).toMatchObject({ birthdayMonth: 12, birthdayDay: 31 });
+            expect(profile).toMatchObject({ birthdayMonth: month, birthdayDay: day });
             expect(profile).not.toHaveProperty('birthdayYear');
         });
 
@@ -167,7 +172,10 @@ describe('FetchUserProfile namespace', () => {
             ['an all-zero value', [0, 0, 0, 0]],
             ['an out-of-range month', [0x07, 0xd0, 13, 1]],
             ['an out-of-range day', [0x07, 0xd0, 1, 32]],
+            ['April 31', [0x07, 0xd0, 4, 31]],
+            ['a leap day in 1900', [0x07, 0x6c, 2, 29]],
             ['a truncated value', [0x07, 0xd0, 3]],
+            ['an oversized value', [0x07, 0xd0, 3, 15, 0]],
         ])('leaves the birthday unavailable for %s', async (_case, value) => {
             const sender = makeSender({
                 body: {