Add error handling for timezone conversion

Fixes 500 error when user timezone is invalid or missing.
Now gracefully falls back to UTC if timezone conversion fails.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tasks/models.py

@@ -156,9 +156,13 @@ class Task(models.Model):
             import pytz
 
             # Get current date in user's timezone
+            try:
                 user_tz = pytz.timezone(self.user.timezone)
                 user_now = timezone.now().astimezone(user_tz)
                 user_today = user_now.date()
+            except (pytz.UnknownTimeZoneError, AttributeError):
+                # Fall back to UTC if user timezone is invalid or missing
+                user_today = timezone.now().date()
 
             return self.due_date < user_today
         return False

tasks/views.py

@@ -220,9 +220,13 @@ class DashboardView(View):
 
         # Get today's date in user's timezone
         import pytz
+        try:
             user_tz = pytz.timezone(request.user.timezone)
             user_now = timezone.now().astimezone(user_tz)
             today = user_now.date()
+        except (pytz.UnknownTimeZoneError, AttributeError):
+            # Fall back to UTC if user timezone is invalid or missing
+            today = timezone.now().date()
 
         # Get filter parameters
         current_filter = request.GET.get('filter', 'all')