fix(time): reject nonexistent local times

src/time/src/mcp_server_time/server.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from enum import Enum
 import json
 from typing import Sequence
@@ -92,6 +92,23 @@ def convert_time(
             tzinfo=source_timezone,
         )
 
+        # Attaching ZoneInfo does not validate local wall times. During a
+        # spring-forward transition, times in the skipped interval are silently
+        # assigned the pre-transition offset even though they never occurred.
+        # A UTC round trip normalizes such a value to the first valid local time,
+        # allowing us to reject the invalid input instead of returning a
+        # misleading conversion.
+        round_tripped_source_time = source_time.astimezone(timezone.utc).astimezone(
+            source_timezone
+        )
+        if round_tripped_source_time.replace(tzinfo=None) != source_time.replace(
+            tzinfo=None
+        ):
+            raise ValueError(
+                f"Time {time_str} does not exist in {source_tz} on "
+                f"{source_time.date()} due to a UTC offset transition"
+            )
+
         target_time = source_time.astimezone(target_timezone)
         source_offset = source_time.utcoffset() or timedelta()
         target_offset = target_time.utcoffset() or timedelta()

src/time/test/time_server_test.py

@@ -120,6 +120,37 @@ def test_convert_time_errors(source_tz, time_str, target_tz, expected_error):
         time_server.convert_time(source_tz, time_str, target_tz)
 
 
+def test_convert_time_rejects_nonexistent_dst_time():
+    # New York advances from 01:59 to 03:00 on this date, so 02:30 never
+    # occurs. ZoneInfo otherwise silently assigns it the earlier UTC-05:00
+    # offset and produces a conversion for a nonexistent wall time.
+    with freeze_time("2026-03-08 12:00:00+00:00"):
+        time_server = TimeServer()
+
+        with pytest.raises(
+            ValueError,
+            match=(
+                r"Time 02:30 does not exist in America/New_York on 2026-03-08 "
+                r"due to a UTC offset transition"
+            ),
+        ):
+            time_server.convert_time("America/New_York", "02:30", "UTC")
+
+
+@pytest.mark.parametrize(
+    "time_str,expected_utc",
+    [
+        ("01:30", "2026-03-08T06:30:00+00:00"),
+        ("03:30", "2026-03-08T07:30:00+00:00"),
+    ],
+)
+def test_convert_time_accepts_valid_times_around_dst_gap(time_str, expected_utc):
+    with freeze_time("2026-03-08 12:00:00+00:00"):
+        result = TimeServer().convert_time("America/New_York", time_str, "UTC")
+
+        assert result.target.datetime == expected_utc
+
+
 @pytest.mark.parametrize(
     "test_time,source_tz,time_str,target_tz,expected",
     [