Repository: AllenNeuralDynamics/aind-data-schema

Issue:
last_modified field is losing its timestamp in DocDB
**Describe the bug**
The created and last_modified fields use `datetime` which doesn't require a timezone and somewhere between creation and upload to DocDB the last_modified field is losing it's timezone. 

Also the comment in `metadata.py` about creation/last_modified matching seems to be outdated:

```
​    # We'll set created and last_modified defaults using the root_validator
    # to ensure they're synced on creation
    created: datetime = Field(
        default_factory=datetime.utcnow,
        title="Created",
        description="The utc date and time the data asset created.",
    )
    last_modified: datetime = Field(
        default_factory=datetime.utcnow,
        title="Last Modified",
        description="The utc date and time that the data asset was last modified.",
    )
```

**Expected behavior**
Both fields could be changed to AwareDatetimeWithDefault to enforce timezone usage and their initial value should be set at the same time (or not if that doesn't actually matter)

Blocking #1086

Hints from issue comments:

Solution patch:
diff --git a/src/aind_data_schema/core/metadata.py b/src/aind_data_schema/core/metadata.py
index 9d086545d..e428da660 100644
--- a/src/aind_data_schema/core/metadata.py
+++ b/src/aind_data_schema/core/metadata.py
@@ -3,7 +3,7 @@
 import inspect
 import json
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 from enum import Enum
 from typing import Dict, List, Literal, Optional, get_args
 from uuid import UUID, uuid4
@@ -16,11 +16,12 @@
     SkipValidation,
     ValidationError,
     ValidationInfo,
+    field_serializer,
     field_validator,
     model_validator,
 )
 
-from aind_data_schema.base import AindCoreModel, is_dict_corrupt
+from aind_data_schema.base import AindCoreModel, is_dict_corrupt, AwareDatetimeWithDefault
 from aind_data_schema.core.acquisition import Acquisition
 from aind_data_schema.core.data_description import DataDescription
 from aind_data_schema.core.instrument import Instrument
@@ -83,15 +84,13 @@ class Metadata(AindCoreModel):
         description="Name of the data asset.",
         title="Data Asset Name",
     )
-    # We'll set created and last_modified defaults using the root_validator
-    # to ensure they're synced on creation
-    created: datetime = Field(
-        default_factory=datetime.utcnow,
+    created: AwareDatetimeWithDefault = Field(
+        default_factory=lambda: datetime.now(tz=timezone.utc),
         title="Created",
         description="The utc date and time the data asset created.",
     )
-    last_modified: datetime = Field(
-        default_factory=datetime.utcnow,
+    last_modified: AwareDatetimeWithDefault = Field(
+        default_factory=lambda: datetime.now(tz=timezone.utc),
         title="Last Modified",
         description="The utc date and time that the data asset was last modified.",
     )
@@ -157,6 +156,16 @@ def validate_core_fields(cls, value, info: ValidationInfo):
             core_model = value
         return core_model
 
+    @field_validator("last_modified", mode="after")
+    def validate_last_modified(cls, value, info: ValidationInfo):
+        """Convert last_modified field to UTC from other timezones"""
+        return value.astimezone(timezone.utc)
+
+    @field_serializer("last_modified")
+    def serialize_last_modified(value) -> str:
+        """Serialize last_modified field"""
+        return value.isoformat().replace("+00:00", "Z")
+
     @model_validator(mode="after")
     def validate_metadata(self):
         """Validator for metadata"""


Test patch:
diff --git a/tests/test_metadata.py b/tests/test_metadata.py
index 78f5428df..506ea3d96 100644
--- a/tests/test_metadata.py
+++ b/tests/test_metadata.py
@@ -5,6 +5,7 @@
 import unittest
 from datetime import datetime, time, timezone
 from unittest.mock import MagicMock, call, patch
+import uuid
 
 from aind_data_schema_models.modalities import Modality
 from aind_data_schema_models.organizations import Organization
@@ -536,6 +537,51 @@ def test_create_from_core_jsons_corrupt(self, mock_is_dict_corrupt: MagicMock, m
             any_order=True,
         )
 
+    def test_last_modified(self):
+        """Test that the last_modified field enforces timezones"""
+        m = Metadata.model_construct(
+            name="name",
+            location="location",
+            id=uuid.uuid4(),
+        )
+        m_dict = m.model_dump(by_alias=True)
+
+        # Test that naive datetime is coerced to timezone-aware datetime
+        date = "2022-11-22T08:43:00"
+        date_with_timezone = datetime.fromisoformat(date).astimezone()
+        m_dict["last_modified"] = "2022-11-22T08:43:00"
+        m2 = Metadata(**m_dict)
+        self.assertIsNotNone(m2)
+        self.assertEqual(m2.last_modified, date_with_timezone)
+
+        # Also check that last_modified is now in UTC
+        self.assertEqual(m2.last_modified.tzinfo, timezone.utc)
+
+        # Test that timezone-aware datetime is not coerced
+        date_minus = "2022-11-22T08:43:00-07:00"
+        m_dict["last_modified"] = date_minus
+        m3 = Metadata(**m_dict)
+        self.assertIsNotNone(m3)
+        self.assertEqual(m3.last_modified, datetime.fromisoformat(date_minus))
+
+        # Test that UTC datetime is not coerced
+        date_utc = "2022-11-22T08:43:00+00:00"
+        m_dict["last_modified"] = date_utc
+        m4 = Metadata(**m_dict)
+        self.assertIsNotNone(m4)
+        self.assertEqual(m4.last_modified, datetime.fromisoformat(date_utc))
+
+        def roundtrip_lm(model):
+            """Helper function to roundtrip last_modified field"""
+            model_json = model.model_dump_json(by_alias=True)
+            model_dict = json.loads(model_json)
+            return model_dict["last_modified"]
+
+        # Test that the output looks right
+        self.assertEqual(m.last_modified.isoformat().replace("+00:00", "Z"), roundtrip_lm(m))
+        self.assertEqual("2022-11-22T15:43:00Z", roundtrip_lm(m3))
+        self.assertEqual("2022-11-22T08:43:00Z", roundtrip_lm(m4))
+
 
 if __name__ == "__main__":
     unittest.main()