diff --git a/src/engram/_models/memory.py b/src/engram/_models/memory.py
index b9c4753..0158bef 100644
--- a/src/engram/_models/memory.py
+++ b/src/engram/_models/memory.py
@@ -2,6 +2,7 @@
 
 from collections.abc import Iterator, Sequence
 from dataclasses import dataclass, field
+from datetime import datetime
 from typing import Any, Literal, TypeAlias
 
 
@@ -27,6 +28,8 @@ class StringInput:
     """String input to extract memories from."""
 
     content: str | list[str]
+    created_at: str | datetime | None = None
+    updated_at: str | datetime | None = None
 
 
 @dataclass(slots=True)
@@ -68,7 +71,7 @@ class MessageInput:
 
     role: Literal["user", "assistant", "system", "tool", "developer"]
     content: str = ""
-    created_at: str | None = None
+    created_at: str | datetime | None = None
     tool_call_id: str | None = None
     name: str | None = None
     tool_calls: list[ToolCallInput] | None = None
@@ -76,12 +79,12 @@ class MessageInput:
 
 @dataclass(slots=True)
 class ConversationInput:
-    """Conversation input that bypasses the extraction pipeline."""
+    """Conversation input to extract memories from."""
 
     messages: list[MessageInput]
     metadata: dict[str, Any] | None = None
-    created_at: str | None = None
-    updated_at: str | None = None
+    created_at: str | datetime | None = None
+    updated_at: str | datetime | None = None
 
 
 # Type alias for the input_data argument to memories.add()
diff --git a/src/engram/_serialization/_builders.py b/src/engram/_serialization/_builders.py
index b4e3289..a273c38 100644
--- a/src/engram/_serialization/_builders.py
+++ b/src/engram/_serialization/_builders.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from datetime import UTC, datetime
 from typing import Any, TypeAlias
 
 from .._models import (
@@ -33,10 +34,7 @@ def _serialize_input(input_data: AddInput) -> dict[str, Any]:
     if isinstance(input_data, str):
         return {"string": {"content": [input_data]}}
     if isinstance(input_data, StringInput):
-        if isinstance(input_data.content, list):
-            return {"string": {"content": input_data.content}}
-        else:
-            return {"string": {"content": [input_data.content]}}
+        return _serialize_string_input(input_data)
     if isinstance(input_data, PreExtractedInput):
         items = [{"content": item.content, "topic": item.topic} for item in input_data.items]
         return {"pre_extracted": {"items": items}}
@@ -49,12 +47,31 @@ def _serialize_input(input_data: AddInput) -> dict[str, Any]:
     raise TypeError(f"Unsupported input type: {type(input_data)}")  # pragma: no cover
 
 
+def _serialize_timestamp(value: str | datetime) -> str:
+    """Format a timestamp as RFC 3339 (e.g. "2024-01-01T00:00:00Z")."""
+    if isinstance(value, str):
+        return value
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=UTC)
+    return value.isoformat().replace("+00:00", "Z")
+
+
+def _serialize_string_input(input_data: StringInput) -> dict[str, Any]:
+    content = input_data.content if isinstance(input_data.content, list) else [input_data.content]
+    body: dict[str, Any] = {"content": content}
+    if input_data.created_at is not None:
+        body["created_at"] = _serialize_timestamp(input_data.created_at)
+    if input_data.updated_at is not None:
+        body["updated_at"] = _serialize_timestamp(input_data.updated_at)
+    return {"string": body}
+
+
 def _serialize_conversation_content(content: ConversationInput) -> dict[str, Any]:
     messages = []
     for msg in content.messages:
         m: dict[str, Any] = {"role": msg.role, "content": msg.content}
         if msg.created_at is not None:
-            m["created_at"] = msg.created_at
+            m["created_at"] = _serialize_timestamp(msg.created_at)
         if msg.tool_call_id is not None:
             m["tool_call_id"] = msg.tool_call_id
         if msg.name is not None:
@@ -66,9 +83,9 @@ def _serialize_conversation_content(content: ConversationInput) -> dict[str, Any
     if content.metadata is not None:
         conversation["metadata"] = content.metadata
     if content.created_at is not None:
-        conversation["created_at"] = content.created_at
+        conversation["created_at"] = _serialize_timestamp(content.created_at)
     if content.updated_at is not None:
-        conversation["updated_at"] = content.updated_at
+        conversation["updated_at"] = _serialize_timestamp(content.updated_at)
     return {"conversation": conversation}
 
 
diff --git a/tests/test_serialization.py b/tests/test_serialization.py
index f78da74..4464033 100644
--- a/tests/test_serialization.py
+++ b/tests/test_serialization.py
@@ -1,3 +1,5 @@
+from datetime import UTC, datetime, timedelta, timezone
+
 from engram._models import (
     ConversationInput,
     MessageInput,
@@ -94,6 +96,51 @@ def test_build_add_body_string_content_with_options() -> None:
     }
 
 
+def test_build_add_body_string_content_with_timestamps() -> None:
+    body = build_add_body(
+        StringInput(
+            content="hello world",
+            created_at="2024-01-01T00:00:00Z",
+            updated_at="2024-01-02T00:00:00Z",
+        ),
+        user_id=None,
+        group=None,
+    )
+    assert body == {
+        "input": {
+            "string": {
+                "content": ["hello world"],
+                "created_at": "2024-01-01T00:00:00Z",
+                "updated_at": "2024-01-02T00:00:00Z",
+            },
+        },
+    }
+
+
+def test_build_add_body_string_content_with_datetime_timestamps() -> None:
+    body = build_add_body(
+        StringInput(
+            content="hello world",
+            created_at=datetime(2024, 1, 1, tzinfo=UTC),
+            updated_at=datetime(2024, 1, 2, tzinfo=timezone(timedelta(hours=-5))),
+        ),
+        user_id=None,
+        group=None,
+    )
+    string_body = body["input"]["string"]
+    assert string_body["created_at"] == "2024-01-01T00:00:00Z"
+    assert string_body["updated_at"] == "2024-01-02T00:00:00-05:00"
+
+
+def test_build_add_body_string_content_naive_datetime_assumed_utc() -> None:
+    body = build_add_body(
+        StringInput(content="hello", created_at=datetime(2024, 1, 1)),
+        user_id=None,
+        group=None,
+    )
+    assert body["input"]["string"]["created_at"] == "2024-01-01T00:00:00Z"
+
+
 def test_build_add_body_conversation_content() -> None:
     messages = [
         MessageInput(role="user", content="hi"),
@@ -147,6 +194,25 @@ def test_build_add_body_conversation_content_with_message_timestamps() -> None:
     assert "tool_call_metadata" not in msg
 
 
+def test_build_add_body_conversation_content_with_datetime_timestamps() -> None:
+    messages = [
+        MessageInput(role="user", content="hi", created_at=datetime(2024, 1, 1, tzinfo=UTC))
+    ]
+    body = build_add_body(
+        ConversationInput(
+            messages=messages,
+            created_at=datetime(2024, 1, 1, tzinfo=UTC),
+            updated_at=datetime(2024, 1, 2, tzinfo=timezone(timedelta(hours=-5))),
+        ),
+        user_id=None,
+        group=None,
+    )
+    conv = body["input"]["conversation"]
+    assert conv["messages"][0]["created_at"] == "2024-01-01T00:00:00Z"
+    assert conv["created_at"] == "2024-01-01T00:00:00Z"
+    assert conv["updated_at"] == "2024-01-02T00:00:00-05:00"
+
+
 def test_build_add_body_conversation_content_with_tool_calls() -> None:
     messages = [
         MessageInput(