diff --git a/.changeset/brown-snails-pull.md b/.changeset/brown-snails-pull.md
new file mode 100644
index 00000000000..ba61a2c003b
--- /dev/null
+++ b/.changeset/brown-snails-pull.md
@@ -0,0 +1,5 @@
+---
+'@firebase/functions': patch
+---
+
+Fix functions to convert Date objects to an ISO string instead of an empty object.
diff --git a/packages-exp/functions-exp/src/serializer.test.ts b/packages-exp/functions-exp/src/serializer.test.ts
index 0a675cd01f3..8c1709d37c2 100644
--- a/packages-exp/functions-exp/src/serializer.test.ts
+++ b/packages-exp/functions-exp/src/serializer.test.ts
@@ -82,6 +82,18 @@ describe('Serializer', () => {
     expect(decode('hello')).to.equal('hello');
   });
 
+  it('encodes date to ISO string', () => {
+    expect(encode(new Date(1620666095891))).to.equal(
+      '2021-05-10T17:01:35.891Z'
+    );
+  });
+
+  it('decodes date string without modifying it', () => {
+    expect(decode('2021-05-10T17:01:35.891Z')).to.equal(
+      '2021-05-10T17:01:35.891Z'
+    );
+  });
+
   // TODO(klimt): Make this test more interesting once we have a complex type
   // that can be created in JavaScript.
   it('encodes array', () => {
@@ -111,12 +123,14 @@ describe('Serializer', () => {
       encode({
         foo: 1,
         bar: 'hello',
-        baz: [1, 2, 3]
+        baz: [1, 2, 3],
+        date: new Date(1620666095891)
       })
     ).to.deep.equal({
       foo: 1,
       bar: 'hello',
-      baz: [1, 2, 3]
+      baz: [1, 2, 3],
+      date: '2021-05-10T17:01:35.891Z'
     });
   });
 
@@ -132,12 +146,14 @@ describe('Serializer', () => {
             value: '1099511627776',
             '@type': 'type.googleapis.com/google.protobuf.Int64Value'
           }
-        ]
+        ],
+        date: '2021-05-10T17:01:35.891Z'
       })
     ).to.deep.equal({
       foo: 1,
       bar: 'hello',
-      baz: [1, 2, 1099511627776]
+      baz: [1, 2, 1099511627776],
+      date: '2021-05-10T17:01:35.891Z'
     });
   });
 
diff --git a/packages-exp/functions-exp/src/serializer.ts b/packages-exp/functions-exp/src/serializer.ts
index bbe2208b025..e6247ec34d1 100644
--- a/packages-exp/functions-exp/src/serializer.ts
+++ b/packages-exp/functions-exp/src/serializer.ts
@@ -56,6 +56,9 @@ export function encode(data: unknown): unknown {
   if (Object.prototype.toString.call(data) === '[object String]') {
     return data;
   }
+  if (data instanceof Date) {
+    return data.toISOString();
+  }
   if (Array.isArray(data)) {
     return data.map(x => encode(x));
   }
diff --git a/packages/functions/src/serializer.ts b/packages/functions/src/serializer.ts
index 677d22d8804..f41784e45fb 100644
--- a/packages/functions/src/serializer.ts
+++ b/packages/functions/src/serializer.ts
@@ -54,6 +54,9 @@ export class Serializer {
     if (Object.prototype.toString.call(data) === '[object String]') {
       return data;
     }
+    if (data instanceof Date) {
+      return data.toISOString();
+    }
     if (Array.isArray(data)) {
       return data.map(x => this.encode(x));
     }
diff --git a/packages/functions/test/serializer.test.ts b/packages/functions/test/serializer.test.ts
index 3de6d6d0d9c..cbdadff05e5 100644
--- a/packages/functions/test/serializer.test.ts
+++ b/packages/functions/test/serializer.test.ts
@@ -88,6 +88,18 @@ describe('Serializer', () => {
     expect(serializer.decode('hello')).to.equal('hello');
   });
 
+  it('encodes date to ISO string', () => {
+    expect(serializer.encode(new Date(1620666095891))).to.equal(
+      '2021-05-10T17:01:35.891Z'
+    );
+  });
+
+  it('decodes date string without modifying it', () => {
+    expect(serializer.decode('2021-05-10T17:01:35.891Z')).to.equal(
+      '2021-05-10T17:01:35.891Z'
+    );
+  });
+
   // TODO(klimt): Make this test more interesting once we have a complex type
   // that can be created in JavaScript.
   it('encodes array', () => {
@@ -117,12 +129,14 @@ describe('Serializer', () => {
       serializer.encode({
         foo: 1,
         bar: 'hello',
-        baz: [1, 2, 3]
+        baz: [1, 2, 3],
+        date: new Date(1620666095891)
       })
     ).to.deep.equal({
       foo: 1,
       bar: 'hello',
-      baz: [1, 2, 3]
+      baz: [1, 2, 3],
+      date: '2021-05-10T17:01:35.891Z'
     });
   });
 
@@ -138,12 +152,14 @@ describe('Serializer', () => {
             value: '1099511627776',
             '@type': 'type.googleapis.com/google.protobuf.Int64Value'
           }
-        ]
+        ],
+        date: '2021-05-10T17:01:35.891Z'
       })
     ).to.deep.equal({
       foo: 1,
       bar: 'hello',
-      baz: [1, 2, 1099511627776]
+      baz: [1, 2, 1099511627776],
+      date: '2021-05-10T17:01:35.891Z'
     });
   });