diff --git a/keycloak_mcp/server.py b/keycloak_mcp/server.py
index eb32462..6e971b9 100644
--- a/keycloak_mcp/server.py
+++ b/keycloak_mcp/server.py
@@ -10,7 +10,7 @@
 import sys
 from collections import Counter
 from collections.abc import Callable
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 
 from mcp.server.fastmcp import FastMCP
 
@@ -22,8 +22,8 @@ def _format_ts(epoch_ms: int | str) -> str:
     """Convert epoch milliseconds to local datetime string."""
     try:
         ts = int(epoch_ms) / 1000
-        return datetime.fromtimestamp(ts).astimezone().strftime("%Y-%m-%d %H:%M:%S")
-    except (ValueError, TypeError, OSError):
+        return datetime.fromtimestamp(ts, tz=timezone.utc).astimezone().strftime("%Y-%m-%d %H:%M:%S")
+    except (ValueError, TypeError):
         return str(epoch_ms)
 
 
@@ -445,13 +445,13 @@ def get_login_stats_by_hour(date_from: str = "", date_to: str = "") -> str:
     failure_by_hour: Counter[int] = Counter()
     for e in success:
         try:
-            hour = datetime.fromtimestamp(int(e["time"]) / 1000).astimezone().hour
+            hour = datetime.fromtimestamp(int(e["time"]) / 1000, tz=timezone.utc).astimezone().hour
             success_by_hour[hour] += 1
         except (KeyError, ValueError, TypeError):
             pass
     for e in failure:
         try:
-            hour = datetime.fromtimestamp(int(e["time"]) / 1000).astimezone().hour
+            hour = datetime.fromtimestamp(int(e["time"]) / 1000, tz=timezone.utc).astimezone().hour
             failure_by_hour[hour] += 1
         except (KeyError, ValueError, TypeError):
             pass
diff --git a/tests/test_server.py b/tests/test_server.py
index e72c102..4bd8b6e 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -1,11 +1,8 @@
 """Tests for MCP server tools."""
 
-import sys
 from datetime import datetime, timedelta
 from unittest.mock import patch
 
-import pytest
-
 from keycloak_mcp import server
 
 SAMPLE_USER = {
@@ -27,10 +24,6 @@ def test_valid_epoch_ms(self):
     def test_invalid_value(self):
         assert server._format_ts("invalid") == "invalid"
 
-    @pytest.mark.skipif(
-        sys.platform == "win32",
-        reason="Windows raises OSError on datetime.fromtimestamp(0); _format_ts falls back to str(0)",
-    )
     def test_zero(self):
         result = server._format_ts(0)
         assert "1970" in result or "1969" in result  # depends on TZ