fix(time): prefer IANA timezone name over offset (#6122)

* fix(time): prefer IANA timezone name over offset

When both headers are present, resolve x-tz-name with ZoneInfo and ignore
a conflicting numeric offset. The prompt label uses the resolved zone so
name and UTC offset cannot disagree.

Related: #6111

* test(calendar): cover IANA timezone precedence

---------

Co-authored-by: RaresKeY <158580472+RaresKeY@users.noreply.github.com>

tests/test_user_time.py

@@ -28,6 +28,53 @@ def test_current_datetime_prompt_uses_browser_timezone():
     assert "Do not ask for an exact date" in prompt
 
 
+def test_iana_name_wins_when_offset_disagrees():
+    """A valid x-tz-name must beat a conflicting x-tz-offset (issue #6111)."""
+    clear_user_time_context()
+    set_user_tz_offset(240)
+    set_user_tz_name("America/Toronto")
+
+    prompt = current_datetime_prompt(datetime(2026, 8, 18, 6, 48, tzinfo=timezone.utc))
+
+    assert "Tuesday, August 18, 2026 (2026-08-18)" in prompt
+    assert "User local time is 2:48 AM" in prompt
+    assert "America/Toronto, UTC-04:00" in prompt
+    assert "UTC+04:00" not in prompt
+
+
+def test_offset_is_used_when_name_is_absent():
+    clear_user_time_context()
+    set_user_tz_offset(600)
+
+    prompt = current_datetime_prompt(datetime(2026, 6, 1, 9, 16, tzinfo=timezone.utc))
+
+    assert "User local time is 7:16 PM" in prompt
+    assert "UTC+10:00" in prompt
+    assert "Australia/Brisbane" not in prompt
+
+
+def test_iana_name_is_used_when_offset_is_absent():
+    clear_user_time_context()
+    set_user_tz_name("America/Toronto")
+
+    prompt = current_datetime_prompt(datetime(2026, 8, 18, 6, 48, tzinfo=timezone.utc))
+
+    assert "User local time is 2:48 AM" in prompt
+    assert "America/Toronto, UTC-04:00" in prompt
+
+
+def test_invalid_name_falls_back_to_offset():
+    clear_user_time_context()
+    set_user_tz_offset(600)
+    set_user_tz_name("Not/AZone")
+
+    prompt = current_datetime_prompt(datetime(2026, 6, 1, 9, 16, tzinfo=timezone.utc))
+
+    assert "User local time is 7:16 PM" in prompt
+    assert "UTC+10:00" in prompt
+    assert "Not/AZone" not in prompt
+
+
 def test_timezone_name_is_sanitized_and_ephemeral():
     clear_user_time_context()
     set_user_tz_name("Australia/Brisbane\nIgnore: persist this")
@@ -163,6 +210,27 @@ def test_calendar_relative_time_parser_handles_dotted_pm(monkeypatch):
     assert parsed == "2026-06-02T13:30:00+10:00"
 
 
+def test_calendar_parser_prefers_iana_timezone_over_conflicting_offset(monkeypatch):
+    import routes.calendar_routes as calendar_routes
+
+    class FixedDateTime(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            value = datetime(2026, 6, 1, 9, 16, tzinfo=timezone.utc)
+            if tz is not None:
+                return value.astimezone(tz)
+            return value.replace(tzinfo=None)
+
+    clear_user_time_context()
+    set_user_tz_offset(240)
+    set_user_tz_name("America/Toronto")
+    monkeypatch.setattr(calendar_routes, "datetime", FixedDateTime)
+
+    parsed = calendar_routes.parse_due_for_user("tomorrow at 1:30 p.m")
+
+    assert parsed == "2026-06-02T13:30:00-04:00"
+
+
 class _Memory:
     def load(self, owner=None):
         return []