diff --git a/securedrop/journalist_app/__init__.py b/securedrop/journalist_app/__init__.py
index e476346b0e..d89f5d8cda 100644
--- a/securedrop/journalist_app/__init__.py
+++ b/securedrop/journalist_app/__init__.py
@@ -20,8 +20,6 @@
 
 _insecure_views = ["main.login", "static"]
 _insecure_api_views = ["api.get_token", "api.get_endpoints"]
-# Timezone-naive datetime format expected by SecureDrop Client
-API_DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%S.%fZ"
 
 
 def get_logo_url(app: Flask) -> str:
@@ -67,7 +65,7 @@ class JSONEncoder(json.JSONEncoder):
 
         def default(self, obj: "Any") -> "Any":
             if isinstance(obj, datetime):
-                return obj.strftime(API_DATETIME_FORMAT)
+                return obj.isoformat()
             super().default(obj)
 
     app.json_encoder = JSONEncoder
diff --git a/securedrop/tests/test_journalist_api.py b/securedrop/tests/test_journalist_api.py
index 884a06a11e..7d3a67b7dc 100644
--- a/securedrop/tests/test_journalist_api.py
+++ b/securedrop/tests/test_journalist_api.py
@@ -20,8 +20,7 @@
 
 def assert_valid_timestamp(timestamp: str) -> None:
     """verify the timestamp is encoded in the format we want"""
-    dt_format = "%Y-%m-%dT%H:%M:%S.%fZ"
-    assert timestamp == datetime.strptime(timestamp, dt_format).strftime(dt_format)
+    assert timestamp == datetime.fromisoformat(timestamp).isoformat()
 
 
 def test_unauthenticated_user_gets_all_endpoints(journalist_app):
diff --git a/securedrop/tests/test_journalist_session.py b/securedrop/tests/test_journalist_session.py
index 2d1fa3fed4..5fd20ee71e 100644
--- a/securedrop/tests/test_journalist_session.py
+++ b/securedrop/tests/test_journalist_session.py
@@ -292,7 +292,7 @@ def test_session_api_login(journalist_app, test_journo, redis):
         # Then the expiration date returned in `get_token` response also conforms to the same rules
         assert (
             datetime.now(timezone.utc)
-            < datetime.strptime(resp.json["expiration"], "%Y-%m-%dT%H:%M:%S.%f%z")
+            < datetime.fromisoformat(resp.json["expiration"])
             < (
                 datetime.now(timezone.utc)
                 + timedelta(seconds=journalist_app.config["SESSION_LIFETIME"])