diff --git a/barte/models.py b/barte/models.py
index 9f4f3c5..6aa7aae 100644
--- a/barte/models.py
+++ b/barte/models.py
@@ -2,11 +2,12 @@
 from datetime import datetime
 from typing import Optional, List, Dict, Any
 from dacite import Config
+from dateutil.parser import parse as parse_date
 
 # Default config for dacite with datetime conversion
 DACITE_CONFIG = Config(
     type_hooks={
-        datetime: lambda x: datetime.fromisoformat(x.replace("Z", "+00:00")) if isinstance(x, str) else x
+        datetime: lambda x: parse_date(x) if isinstance(x, str) else x
     }
 )
 
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 6dc16e9..4abcdde 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,3 +1,4 @@
 pytest>=7.0.0
 pytest-cov>=4.0.0
-requests-mock>=1.10.0 
\ No newline at end of file
+requests-mock>=1.10.0 
+python-dateutil>=2.8.0
\ No newline at end of file
diff --git a/setup.py b/setup.py
index 48954a4..b866ad7 100644
--- a/setup.py
+++ b/setup.py
@@ -9,7 +9,8 @@
     packages=find_packages(),
     install_requires=[
         "requests>=2.25.0",
-        "dacite>=1.8.0"
+        "dacite>=1.8.0",
+        "python-dateutil>=2.8.0"
     ],
     python_requires=">=3.11",
     classifiers=[
diff --git a/tests/test_client.py b/tests/test_client.py
index ad1ca98..7800c16 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -316,6 +316,14 @@ def test_pix_charge_get_qrcode(self, mock_get, mock_charge_response):
 
     def test_client_singleton(self):
         """Test client singleton pattern"""
+        # Reset singleton for initial state
+        BarteClient._instance = None
+
+        # Test uninitialized state
+        with pytest.raises(RuntimeError) as exc_info:
+            BarteClient.get_instance()
+        assert "BarteClient not initialized" in str(exc_info.value)
+
         # First initialization
         client1 = BarteClient(api_key="test_key", environment="sandbox")
         assert BarteClient.get_instance() == client1