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>

Author: uadhran

src/user_time.py

@@ -8,7 +8,7 @@
 
 import re
 from contextvars import ContextVar
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timedelta, timezone, tzinfo
 from typing import Dict, Optional
 
 
@@ -65,19 +65,31 @@ def format_utc_offset(offset_min: Optional[int]) -> str:
     return f"{sign}{hours:02d}:{minutes:02d}"
 
 
-def user_timezone() -> timezone:
-    """Return the best known user timezone as a fixed-offset tzinfo."""
+def _zoneinfo_from_name():
+    """Return ZoneInfo for the request's IANA name, or None if missing/invalid."""
+    name = get_user_tz_name()
+    if not name:
+        return None
+    try:
+        from zoneinfo import ZoneInfo
+        return ZoneInfo(name)
+    except Exception:
+        return None
+
+
+def user_timezone() -> tzinfo:
+    """Return the best known user timezone.
+
+    A valid IANA name wins over x-tz-offset. The offset is a fixed number and
+    can disagree with the name (wrong sign, stale client); the name carries DST.
+    """
+    zone = _zoneinfo_from_name()
+    if zone is not None:
+        return zone
     offset = get_user_tz_offset()
-    if offset is None:
-        name = get_user_tz_name()
-        if name:
-            try:
-                from zoneinfo import ZoneInfo
-                return ZoneInfo(name)
-            except Exception:
-                pass
-        return datetime.now().astimezone().tzinfo or timezone.utc
-    return timezone(timedelta(minutes=offset))
+    if offset is not None:
+        return timezone(timedelta(minutes=offset))
+    return datetime.now().astimezone().tzinfo or timezone.utc
 
 
 def now_user_local(now_utc: Optional[datetime] = None) -> datetime:
@@ -100,14 +112,13 @@ def _clock_label(dt: datetime) -> str:
 
 def timezone_label(dt: Optional[datetime] = None) -> str:
     """Return a concise display label such as Australia/Brisbane, UTC+10:00."""
-    offset = get_user_tz_offset()
-    if offset is None:
-        if dt is None:
-            dt = datetime.now().astimezone()
-        offset = int((dt.utcoffset() or timedelta()).total_seconds() // 60)
+    if dt is None:
+        dt = now_user_local()
+    offset = int((dt.utcoffset() or timedelta()).total_seconds() // 60)
     offset_label = f"UTC{format_utc_offset(offset)}"
-    name = get_user_tz_name()
-    return f"{name}, {offset_label}" if name else offset_label
+    if _zoneinfo_from_name() is not None:
+        return f"{get_user_tz_name()}, {offset_label}"
+    return offset_label
 
 
 def current_datetime_prompt(now_utc: Optional[datetime] = None) -> str:

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 now(cls, tz=None):
     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 []