How to Resolve "TypeError: Object of type datetime is not JSON serializable" in Python

When working with Python's json library, you may encounter the TypeError: Object of type datetime is not JSON serializable. This error occurs because the JSON data format has a limited set of supported data types (strings, numbers, booleans, lists, and dictionaries) and does not have a native equivalent for a datetime object.

To serialize a datetime object, you must first convert it into a string, which is a JSON-supported type. This guide will demonstrate the three most common and effective ways to handle this conversion, from a simple catch-all solution to more explicit formatting methods.

Understanding the Error: The JSON Data Type Limitation

The JSON (JavaScript Object Notation) standard is designed to be a simple, text-based data interchange format. It natively supports only the following data types:

• String: "hello"
• Number: 123, 45.67
• Boolean: true, false
• Null: null
• Array: [1, "a", true] (a Python list)
• Object: {"key": "value"} (a Python dict)

Python's datetime object is not on this list. When json.dumps() encounters an object it doesn't know how to serialize, it raises a TypeError.

Reproducing the TypeError

The error is triggered when you try to serialize a dictionary (or other object) that contains a datetime object.

Example of code causing the error:

import json
from datetime import datetime

data_to_serialize = {
    'created_at': datetime.now(),
    'user': 'Tom'
}

json_string = json.dumps(data_to_serialize)

Output:

Traceback (most recent call last):
  File "main.py", line 10, in <module>
    json_string = json.dumps(data_to_serialize)
  File "/usr/lib/python3.8/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3.8/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.8/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python3.8/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type datetime is not JSON serializable

Solution 1: Use the default Parameter in json.dumps()

The json.dumps() function has a convenient default parameter. You can pass it a function that will be called for any object that the JSON encoder doesn't know how to handle. This provides a clean, "catch-all" way to serialize datetime objects.

Solution:

import json
from datetime import datetime

data_to_serialize = {
    'created_at': datetime.now(),
    'user': 'Tom'
}

json_string = json.dumps(data_to_serialize, default=str)

print(json_string)

Output:

{"created_at": "2025-11-16 14:38:02.691996", "user": "Tom"}

Solution 2: Convert to a Standard String Format (Best Practice)

For interoperability with other systems and APIs, it is a best practice to convert datetime objects to a standard, unambiguous string format like ISO 8601. The datetime.isoformat() method is perfect for this.

Solution:

import json
from datetime import datetime

now = datetime.now()

data_to_serialize = {
    'created_at': now.isoformat(),
    'user': 'Tom'
}

json_string = json.dumps(data_to_serialize)
print(f"Serialized with isoformat(): {json_string}")

custom_format_str = now.strftime("%Y-%m-%d %H:%M")
data_to_serialize_custom = {
    'created_at': custom_format_str,
    'user': 'Tom'
}
json_string_custom = json.dumps(data_to_serialize_custom)
print(f"Serialized with strftime(): {json_string_custom}")

Output:

Serialized with isoformat(): {"created_at": "2025-11-16T14:38:27.134519", "user": "Tom"}
Serialized with strftime(): {"created_at": "2025-11-16 14:38", "user": "Tom"}

Solution 3: Manually Convert with str()

A quick and simple fix is to manually wrap the datetime object with the str() function before passing it to json.dumps(). This is essentially what the default=str parameter does, but applied on a case-by-case basis.

Solution:

import json
from datetime import datetime

now = datetime.now()

data_to_serialize = {
    'created_at': str(now),
    'user': 'Tom'
}

json_string = json.dumps(data_to_serialize)
print(json_string)

Output:

{"created_at": "2025-11-16 14:38:38.671466", "user": "Tom"}

Conclusion

The TypeError: Object of type datetime is not JSON serializable is a straightforward problem to solve. By converting your datetime objects to a string format before serialization, you can easily bridge the gap between Python's data types and the JSON standard.