Changes Made

1. config/schema.sql — Added timezone_name VARCHAR(100) to user_preferences table + an ALTER TABLE migration at the bottom for existing DBs.
  2. core/tz.py — Rewrote with dual-path timezone support:
  - Request context: _get_request_tz() now checks X-Timezone-Name header (IANA) first, falls back to X-Timezone-Offset
  - Background jobs: New tz_for_user(user_uuid) and user_now_for(user_uuid) read stored timezone_name from prefs, fall back to numeric offset, then UTC
  - All existing function signatures (user_now(), user_today()) preserved for backward compat

  3. scheduler/daemon.py — Fixed 3 bugs:
  - _user_now_for() now delegates to tz.user_now_for() which uses IANA timezone names (DST-safe)
  - check_nagging() — replaced datetime.utcnow() with _user_now_for(user_uuid) so nags evaluate in user's timezone
  - poll_callback() — replaced single UTC midnight check with _check_per_user_midnight_schedules() that iterates users and creates daily schedules at their local midnight

  4. api/routes/preferences.py — Added "timezone_name" to allowed PUT fields.

  5. synculous-client/src/lib/api.ts — Added X-Timezone-Name header to every request + added timezone_name to preferences update type.

  6. synculous-client/src/app/dashboard/layout.tsx — Now syncs both timezone_offset and timezone_name (via Intl.DateTimeFormat().resolvedOptions().timeZone) on session start.

core/tz.py

@@ -1,12 +1,36 @@
 """
 core/tz.py - Timezone-aware date/time helpers
 
-The frontend sends X-Timezone-Offset (minutes from UTC, same sign as
-JavaScript's getTimezoneOffset — positive means behind UTC).
-These helpers convert server UTC to the user's local date/time.
+The frontend sends:
+  X-Timezone-Name   – IANA timezone (e.g. "America/Chicago"), preferred
+  X-Timezone-Offset – minutes from UTC (JS getTimezoneOffset sign), fallback
+
+For background tasks (no request context) the scheduler reads the stored
+timezone_name / timezone_offset from user_preferences.
 """
 
 from datetime import datetime, date, timezone, timedelta
+from zoneinfo import ZoneInfo
+
+import core.postgres as postgres
+
+# ── Request-context helpers (used by Flask route handlers) ────────────
+
+def _get_request_tz():
+    """Return a tzinfo from the current Flask request headers.
+    Prefers X-Timezone-Name (IANA), falls back to X-Timezone-Offset."""
+    try:
+        import flask
+        name = flask.request.headers.get("X-Timezone-Name")
+        if name:
+            try:
+                return ZoneInfo(name)
+            except (KeyError, Exception):
+                pass
+        offset = int(flask.request.headers.get("X-Timezone-Offset", 0))
+        return timezone(timedelta(minutes=-offset))
+    except (ValueError, TypeError, RuntimeError):
+        return timezone.utc
 
 
 def _get_offset_minutes():
@@ -16,7 +40,6 @@ def _get_offset_minutes():
         import flask
         return int(flask.request.headers.get("X-Timezone-Offset", 0))
     except (ValueError, TypeError, RuntimeError):
-        # RuntimeError: outside of request context
         return 0
 
 
@@ -26,14 +49,45 @@ def _offset_to_tz(offset_minutes):
 
 
 def user_now(offset_minutes=None):
-    """Current datetime in the user's timezone.
-    If offset_minutes is provided, uses that instead of the request header."""
-    if offset_minutes is None:
-        offset_minutes = _get_offset_minutes()
-    tz = _offset_to_tz(offset_minutes)
+    """Current datetime in the user's timezone (request-context).
+    If offset_minutes is provided, uses that directly.
+    Otherwise reads request headers (prefers IANA name over offset)."""
+    if offset_minutes is not None:
+        tz = _offset_to_tz(offset_minutes)
+    else:
+        tz = _get_request_tz()
     return datetime.now(tz)
 
 
 def user_today(offset_minutes=None):
     """Current date in the user's timezone."""
     return user_now(offset_minutes).date()
+
+
+# ── Stored-preference helpers (used by scheduler / background jobs) ───
+
+def tz_for_user(user_uuid):
+    """Return a tzinfo for *user_uuid* from stored preferences.
+    Priority: timezone_name (IANA) > timezone_offset (minutes) > UTC."""
+    prefs = postgres.select_one("user_preferences", {"user_uuid": user_uuid})
+    if prefs:
+        name = prefs.get("timezone_name")
+        if name:
+            try:
+                return ZoneInfo(name)
+            except (KeyError, Exception):
+                pass
+        offset = prefs.get("timezone_offset")
+        if offset is not None:
+            return timezone(timedelta(minutes=-offset))
+    return timezone.utc
+
+
+def user_now_for(user_uuid):
+    """Current datetime in a user's timezone using their stored preferences."""
+    return datetime.now(tz_for_user(user_uuid))
+
+
+def user_today_for(user_uuid):
+    """Current date in a user's timezone using their stored preferences."""
+    return user_now_for(user_uuid).date()