diff --git a/aleph_message/models/__init__.py b/aleph_message/models/__init__.py
index 9306012..8afec37 100644
--- a/aleph_message/models/__init__.py
+++ b/aleph_message/models/__init__.py
@@ -1,3 +1,4 @@
+import datetime
 import json
 from copy import copy
 from enum import Enum
@@ -11,7 +12,7 @@
     Literal,
     Optional,
     Type,
-    Union,
+    Union, TypeVar, cast,
 )
 
 from pydantic import BaseModel, Extra, Field, validator
@@ -203,7 +204,7 @@ class BaseMessage(BaseModel):
     size: Optional[int] = Field(
         default=None, description="Size of the content"
     )  # Almost always present
-    time: float = Field(description="Unix timestamp when the message was published")
+    time: datetime.datetime = Field(description="Unix timestamp or datetime when the message was published")
     item_type: ItemType = Field(description="Storage method used for the content")
     item_content: Optional[str] = Field(
         default=None,
@@ -265,6 +266,13 @@ def check_confirmed(cls, v, values):
             raise ValueError("Message cannot be 'confirmed' without 'confirmations'")
         return v
 
+    @validator("time")
+    def convert_float_to_datetime(cls, v, values):
+        if isinstance(v, float):
+            v = datetime.datetime.fromtimestamp(v)
+        assert isinstance(v, datetime.datetime)
+        return v
+
     class Config:
         extra = Extra.forbid
         exclude = {"id_", "_id"}
@@ -334,14 +342,10 @@ class InstanceMessage(BaseMessage):
     ForgetMessage,
 ]
 
-AlephMessageType: TypeAlias = Union[
-    Type[PostMessage],
-    Type[AggregateMessage],
-    Type[StoreMessage],
-    Type[ProgramMessage],
-    Type[InstanceMessage],
-    Type[ForgetMessage],
-]
+
+T = TypeVar('T', bound=AlephMessage)
+
+AlephMessageType: TypeAlias = Type[T]
 
 message_classes: List[AlephMessageType] = [
     PostMessage,
@@ -383,16 +387,16 @@ def add_item_content_and_hash(message_dict: Dict, inplace: bool = False):
 
 def create_new_message(
     message_dict: Dict,
-    factory: Optional[AlephMessageType] = None,
-) -> AlephMessage:
+    factory: Optional[Type[T]] = None,
+) -> T:
     """Create a new message from a dict.
     Computes the 'item_content' and 'item_hash' fields.
     """
     message_content = add_item_content_and_hash(message_dict)
     if factory:
-        return factory.parse_obj(message_content)
+        return cast(T, factory.parse_obj(message_content))
     else:
-        return parse_message(message_content)
+        return cast(T, parse_message(message_content))
 
 
 def create_message_from_json(
diff --git a/aleph_message/tests/test_models.py b/aleph_message/tests/test_models.py
index ce170bf..48d587a 100644
--- a/aleph_message/tests/test_models.py
+++ b/aleph_message/tests/test_models.py
@@ -25,7 +25,7 @@
     create_message_from_file,
     create_message_from_json,
     create_new_message,
-    parse_message,
+    parse_message, AlephMessage,
 )
 from aleph_message.tests.download_messages import MESSAGES_STORAGE_PATH
 
@@ -87,7 +87,7 @@ def test_messages_last_page():
         if message_dict["item_hash"] in HASHES_TO_IGNORE:
             continue
 
-        message = parse_message(message_dict)
+        message: AlephMessage = parse_message(message_dict)
         assert message
 
 
@@ -285,9 +285,14 @@ def test_create_new_message():
         "signature": "0x123456789",  # Signature validation requires using aleph-client
     }
 
-    new_message_1 = create_new_message(message_dict, factory=PostMessage)
+    new_message_1: PostMessage = create_new_message(message_dict, factory=PostMessage)
     assert new_message_1
     assert new_message_1.type == MessageType.post
+    # Check that the time was converted to a datetime
+    assert new_message_1.time.isoformat() == '2021-07-07T10:04:47.017000+00:00'
+
+    # The time field can be either a float or a datetime as string
+    message_dict["time"] = '2021-07-07T10:04:47.017000+00:00'
     new_message_2 = create_message_from_json(
         json.dumps(message_dict), factory=PostMessage
     )
@@ -303,7 +308,7 @@ def test_messages_from_disk():
             data_dict = json.load(page_fd)
         for message_dict in data_dict["messages"]:
             try:
-                message = parse_message(message_dict)
+                message: AlephMessage = parse_message(message_dict)
                 assert message
             except ValidationError as e:
                 console.print("-" * 79)
diff --git a/requirements.txt b/requirements.txt
deleted file mode 100644
index d5a2c3a..0000000
--- a/requirements.txt
+++ /dev/null
@@ -1,18 +0,0 @@
-attrs==22.2.0
-certifi==2022.12.7
-charset-normalizer==3.0.1
-exceptiongroup==1.1.0
-idna==3.4
-iniconfig==2.0.0
-markdown-it-py==2.2.0
-mdurl==0.1.2
-packaging==23.0
-pluggy==1.0.0
-pydantic==1.10.5
-Pygments==2.14.0
-pytest==7.2.1
-requests==2.28.2
-rich==13.3.1
-tomli==2.0.1
-typing-extensions==4.5.0
-urllib3==1.26.14