diff --git a/CHANGELOG.md b/CHANGELOG.md
index ff4e0cf..949e1a6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Fixed
+
+- Don't error when there's a datetime in the header https://github.com/OpenDataServices/flatten-tool/issues/438
+
 ## [0.24.1] - 2024-02-09
 
 ### Fixed
diff --git a/flattentool/__init__.py b/flattentool/__init__.py
index 004afb8..803ef76 100644
--- a/flattentool/__init__.py
+++ b/flattentool/__init__.py
@@ -1,4 +1,5 @@
 import codecs
+import datetime
 import json
 import sys
 from collections import OrderedDict
@@ -192,12 +193,14 @@ def __float__(self):
         return self
 
 
-def decimal_default(o):
+def decimal_datetime_default(o):
     if isinstance(o, Decimal):
         if int(o) == o:
             return int(o)
         else:
             return NumberStr(o)
+    if isinstance(o, datetime.datetime):
+        return str(o)
     raise TypeError(repr(o) + " is not JSON serializable")
 
 
@@ -372,12 +375,18 @@ def unflatten(
     else:
         if output_name is None:
             print(
-                json.dumps(base, indent=4, default=decimal_default, ensure_ascii=False)
+                json.dumps(
+                    base, indent=4, default=decimal_datetime_default, ensure_ascii=False
+                )
             )
         else:
             with codecs.open(output_name, "w", encoding="utf-8") as fp:
                 json.dump(
-                    base, fp, indent=4, default=decimal_default, ensure_ascii=False
+                    base,
+                    fp,
+                    indent=4,
+                    default=decimal_datetime_default,
+                    ensure_ascii=False,
                 )
     if cell_source_map:
         with codecs.open(cell_source_map, "w", encoding="utf-8") as fp:
@@ -385,7 +394,7 @@ def unflatten(
                 cell_source_map_data,
                 fp,
                 indent=4,
-                default=decimal_default,
+                default=decimal_datetime_default,
                 ensure_ascii=False,
             )
     if heading_source_map:
@@ -394,6 +403,6 @@ def unflatten(
                 heading_source_map_data,
                 fp,
                 indent=4,
-                default=decimal_default,
+                default=decimal_datetime_default,
                 ensure_ascii=False,
             )
diff --git a/flattentool/tests/test_init.py b/flattentool/tests/test_init.py
index 511404d..0f382dc 100644
--- a/flattentool/tests/test_init.py
+++ b/flattentool/tests/test_init.py
@@ -1,12 +1,13 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import datetime
 import json
 from decimal import Decimal
 
 import pytest
 
-from flattentool import decimal_default, unflatten
+from flattentool import decimal_datetime_default, unflatten
 
 
 def original_cell_and_row_locations(data):
@@ -51,9 +52,13 @@ def original_headings(heading_data):
     return headings
 
 
-def test_decimal_default():
-    assert json.dumps(Decimal("1.2"), default=decimal_default) == "1.2"
-    assert json.dumps(Decimal("42"), default=decimal_default) == "42"
+def test_decimal_datetime_default():
+    assert json.dumps(Decimal("1.2"), default=decimal_datetime_default) == "1.2"
+    assert json.dumps(Decimal("42"), default=decimal_datetime_default) == "42"
+    assert (
+        json.dumps(datetime.datetime(2024, 1, 1), default=decimal_datetime_default)
+        == '"2024-01-01 00:00:00"'
+    )
 
 
 def lines_strip_whitespace(text):