diff --git a/gitlabform/util.py b/gitlabform/util.py
index e4a70d599..77fa6f7c1 100644
--- a/gitlabform/util.py
+++ b/gitlabform/util.py
@@ -4,4 +4,4 @@
 def to_str(a_dict: dict) -> str:
     # arguably the most readable form of a dict in a single line
     # is JSON with sorted keys
-    return json.dumps(a_dict, sort_keys=True)
+    return json.dumps(a_dict, sort_keys=True, default=str)
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
new file mode 100644
index 000000000..820528afc
--- /dev/null
+++ b/tests/unit/test_utils.py
@@ -0,0 +1,20 @@
+import datetime
+
+from gitlabform.util import to_str
+
+
+def test_date_stringification():
+    example_project_config = {
+        "members": {
+            "enforce": True,
+            "users": {
+                "project_204_bot": {"access_level": 40},
+                "project_204_bot_211c9f071a88910e7ed518cc5d81436a": {
+                    "access_level": 40,
+                    "expires_at": datetime.date(2024, 12, 12),
+                },
+            },
+        }
+    }
+    output = to_str(example_project_config)
+    assert type(output) is str