diff --git a/src/log_analyzer_cli/analyzer.py b/src/log_analyzer_cli/analyzer.py
index cac046a..c4a4e4a 100644
--- a/src/log_analyzer_cli/analyzer.py
+++ b/src/log_analyzer_cli/analyzer.py
@@ -74,6 +74,13 @@ def analyze(
         Returns:
             Analysis result.
         """
+        # Clear any patterns accumulated by a previous call so re-using the
+        # same LogAnalyzer across multiple batches does not leak earlier
+        # groups into the new result. Callers that want to keep history
+        # across calls should manage their own aggregation instead of
+        # relying on internal instance state.
+        self._error_patterns = {}
+
         result = AnalysisResult()
         result.total_lines = len(entries)
         result.parsed_entries = len(entries)
diff --git a/src/log_analyzer_cli/parsers/syslog.py b/src/log_analyzer_cli/parsers/syslog.py
index 99891f3..50f2243 100644
--- a/src/log_analyzer_cli/parsers/syslog.py
+++ b/src/log_analyzer_cli/parsers/syslog.py
@@ -85,7 +85,6 @@ def parse(self, line: str) -> Optional[ParsedEntry]:
                 groups = match.groupdict()
                 
                 timestamp = self._parse_timestamp(groups.get("timestamp", ""))
-                level = detect_log_level(line)  # Check full line for level
                 
                 metadata = {}
                 if groups.get("host"):
@@ -94,11 +93,12 @@ def parse(self, line: str) -> Optional[ParsedEntry]:
                     metadata["process"] = groups["process"]
                     source = groups["process"]
                 else:
-                    # Use host as source when no process name
                     source = groups.get("host")
                 if groups.get("pid"):
                     metadata["pid"] = groups["pid"]
                 
+                level = detect_log_level(groups.get("message", ""))
+                
                 return ParsedEntry(
                     raw=line,
                     timestamp=timestamp,
diff --git a/src/log_analyzer_cli/utils.py b/src/log_analyzer_cli/utils.py
index 4d518a3..4a0865a 100644
--- a/src/log_analyzer_cli/utils.py
+++ b/src/log_analyzer_cli/utils.py
@@ -41,6 +41,8 @@ def _try_parse_datetime(ts_str: str) -> Optional[datetime]:
         "%Y-%m-%dT%H:%M:%S.%f",
         "%Y-%m-%d %H:%M:%S",
         "%Y-%m-%dT%H:%M:%S",
+        "%Y-%m-%dT%H:%M:%S.%f%z",
+        "%Y-%m-%d %H:%M:%S.%f%z",
         "%Y-%m-%dT%H:%M:%S%z",
         "%d/%b/%Y:%H:%M:%S",
         "%b %d %H:%M:%S",
diff --git a/tests/test_analyzer.py b/tests/test_analyzer.py
index cd4ccf1..950134e 100644
--- a/tests/test_analyzer.py
+++ b/tests/test_analyzer.py
@@ -149,6 +149,31 @@ def test_reset(self):
         analyzer.reset()
         assert len(analyzer._error_patterns) == 0
 
+    def test_repeated_analyze_does_not_leak_error_groups(self):
+        analyzer = LogAnalyzer()
+        first_batch = [
+            ParsedEntry(
+                raw="Error: timeout after 30s",
+                level="ERROR",
+                message="timeout after 30s",
+            ),
+        ]
+        second_batch = [
+            ParsedEntry(
+                raw="Error: db connection lost",
+                level="ERROR",
+                message="db connection lost",
+            ),
+        ]
+
+        first_result = analyzer.analyze(first_batch)
+        second_result = analyzer.analyze(second_batch)
+
+        assert len(first_result.error_groups) == 1
+        assert len(second_result.error_groups) == 1
+        assert second_result.error_groups[0].pattern != first_result.error_groups[0].pattern
+        assert second_result.error_groups[0].count == 1
+
 
 class TestAnalyzeLogEntries:
     """Tests for the analyze_log_entries function."""
diff --git a/tests/test_utils.py b/tests/test_utils.py
new file mode 100644
index 0000000..d261221
--- /dev/null
+++ b/tests/test_utils.py
@@ -0,0 +1,93 @@
+"""Tests for log-analyzer-cli utility functions."""
+
+from __future__ import annotations
+
+from datetime import datetime, timedelta, timezone
+
+from log_analyzer_cli.utils import (
+    _try_parse_datetime,
+    detect_log_level,
+    parse_timestamp,
+)
+
+
+class TestTryParseDatetime:
+    """Tests for _try_parse_datetime."""
+
+    def test_iso_with_microseconds_and_offset_colon(self):
+        # Python's datetime.isoformat() emits exactly this form
+        assert _try_parse_datetime("2026-06-21T19:25:00.123456+02:00") == datetime(
+            2026, 6, 21, 19, 25, 0, 123456,
+            tzinfo=timezone(timedelta(hours=2)),
+        )
+
+    def test_iso_with_microseconds_and_offset_no_colon(self):
+        # ISO 8601 also allows ±HHMM form
+        assert _try_parse_datetime("2026-06-21T19:25:00.123456+0200") == datetime(
+            2026, 6, 21, 19, 25, 0, 123456,
+            tzinfo=timezone(timedelta(hours=2)),
+        )
+
+    def test_space_separator_with_microseconds_and_offset(self):
+        # The space-separator variant also gets a combined format
+        assert _try_parse_datetime("2026-06-21 19:25:00.123456+02:00") == datetime(
+            2026, 6, 21, 19, 25, 0, 123456,
+            tzinfo=timezone(timedelta(hours=2)),
+        )
+
+    def test_iso_microseconds_with_z_suffix(self):
+        # Existing Z-substitution should still work for fractional seconds
+        assert _try_parse_datetime("2026-06-21T19:25:00.123Z") == datetime(
+            2026, 6, 21, 19, 25, 0, 123000,
+            tzinfo=timezone.utc,
+        )
+
+    def test_iso_microseconds_no_timezone(self):
+        # The pre-existing microsecond form must keep working
+        assert _try_parse_datetime("2026-06-21T19:25:00.123456") == datetime(
+            2026, 6, 21, 19, 25, 0, 123456,
+        )
+
+    def test_iso_no_microseconds_with_offset(self):
+        # The pre-existing offset form must keep working
+        assert _try_parse_datetime("2026-06-21T19:25:00+02:00") == datetime(
+            2026, 6, 21, 19, 25, 0,
+            tzinfo=timezone(timedelta(hours=2)),
+        )
+
+    def test_iso_no_microseconds_no_timezone(self):
+        # The pre-existing plain form must keep working
+        assert _try_parse_datetime("2026-06-21T19:25:00") == datetime(
+            2026, 6, 21, 19, 25, 0,
+        )
+
+
+class TestParseTimestampIntegration:
+    """End-to-end checks through parse_timestamp."""
+
+    def test_combined_form_extracts_full_datetime(self):
+        # parse_timestamp's first regex captures the whole ISO timestamp;
+        # the inner _try_parse_datetime must now handle microseconds+offset.
+        assert parse_timestamp(
+            "2026-06-21T19:25:00.123456+02:00 ERROR something failed"
+        ) == datetime(2026, 6, 21, 19, 25, 0, 123456,
+                      tzinfo=timezone(timedelta(hours=2)))
+
+    def test_z_fractional_extracts_full_datetime(self):
+        assert parse_timestamp(
+            "2026-06-21T19:25:00.123Z ERROR something failed"
+        ) == datetime(2026, 6, 21, 19, 25, 0, 123000, tzinfo=timezone.utc)
+
+    def test_plain_form_still_parses(self):
+        # regression guard
+        assert parse_timestamp("2026-06-21 19:25:00 INFO ok") == datetime(
+            2026, 6, 21, 19, 25, 0,
+        )
+
+
+class TestDetectLogLevel:
+    def test_returns_uppercase_level(self):
+        assert detect_log_level("2025-01-01 error: bad") == "ERROR"
+
+    def test_returns_unknown_for_plain_text(self):
+        assert detect_log_level("just a normal line") == "UNKNOWN"