fix: Google Tasks due dates shifted a day earlier in negative-offset timezones

Google Tasks' due field is date-only but always serialized as an
RFC3339 timestamp fixed at midnight UTC regardless of the user's
timezone. Converting that instant into displayTZ (dueDate.In(tz))
reinterpreted it as a real moment in time instead of re-anchoring the
same calendar date to local midnight -- midnight UTC becomes 2pm the
previous day in UTC-10 (Pacific/Honolulu), so every dated Google Task
silently landed one day earlier than its actual due date. A task due
tomorrow showed up in today's section.

Fix: extract the Y/M/D from the UTC-anchored timestamp (which IS the
intended calendar date) and rebuild midnight in displayTZ from those
components, instead of converting the instant.

2 files changed, 53 insertions, 2 deletions

diff --git a/internal/api/google_tasks.go b/internal/api/google_tasks.go
index b580be4..1f9aefb 100644
--- a/internal/api/google_tasks.go
+++ b/internal/api/google_tasks.go
@@ -95,14 +95,25 @@ func (c *GoogleTasksClient) getTasksFromList(ctx context.Context, listID string)
 
 		// Parse due date if present (RFC3339 format, but date-only)
 		if item.Due != "" {
-			// Google Tasks due dates are in RFC3339 format but typically just the date part
+			// Google Tasks due dates are date-only, but the API always
+			// serializes them as an RFC3339 timestamp fixed at midnight UTC
+			// regardless of the user's timezone. Converting that instant
+			// into displayTZ (dueDate.In(c.displayTZ)) reinterprets it as a
+			// real moment in time and shifts the calendar date backward for
+			// any timezone behind UTC -- midnight UTC becomes 2pm the
+			// PREVIOUS day in UTC-10, silently moving every dated Google
+			// Task one day earlier than its actual due date. The Y/M/D of
+			// the UTC-anchored timestamp IS the intended calendar date, so
+			// re-anchor those same components to displayTZ midnight instead
+			// of converting the instant.
 			dueDate, err := time.Parse(time.RFC3339, item.Due)
 			if err != nil {
 				// Try date-only format
 				dueDate, err = time.ParseInLocation("2006-01-02", item.Due[:10], c.displayTZ)
 			}
 			if err == nil {
-				dueInTZ := dueDate.In(c.displayTZ)
+				y, m, d := dueDate.Date()
+				dueInTZ := time.Date(y, m, d, 0, 0, 0, 0, c.displayTZ)
 				task.DueDate = &dueInTZ
 			}
 		}
diff --git a/internal/api/google_tasks_test.go b/internal/api/google_tasks_test.go
index fbfdd63..bdb0187 100644
--- a/internal/api/google_tasks_test.go
+++ b/internal/api/google_tasks_test.go
@@ -185,3 +185,43 @@ func TestGetTasksByDateRange_OutOfRangeExcluded(t *testing.T) {
 		t.Errorf("expected task-in-range, got %s", tasks[0].ID)
 	}
 }
+
+// TestGetTasksFromList_DueDateNotShiftedByNegativeOffsetTZ guards against a
+// regression where a task due "tomorrow" showed up "today" on the widget.
+// Google Tasks' due field is date-only but always serialized as midnight
+// UTC. Converting that instant into a negative-offset display timezone
+// (e.g. Pacific/Honolulu, UTC-10) instead of re-anchoring the same calendar
+// date to local midnight shifted every dated task back by one day.
+func TestGetTasksFromList_DueDateNotShiftedByNegativeOffsetTZ(t *testing.T) {
+	honolulu, err := time.LoadLocation("Pacific/Honolulu")
+	if err != nil {
+		t.Skipf("Pacific/Honolulu tzdata not available: %v", err)
+	}
+
+	body := tasksAPIResponse([]map[string]interface{}{
+		{
+			"id":      "task-tomorrow",
+			"title":   "Due tomorrow",
+			"status":  "needsAction",
+			"due":     "2026-07-29T00:00:00.000Z",
+			"updated": "2026-07-28T00:00:00Z",
+		},
+	})
+	server := newTasksServer(body)
+	defer server.Close()
+
+	client := newTestGoogleTasksClient(t, server, "@default", honolulu)
+	tasks, err := client.GetTasks(context.Background())
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(tasks) != 1 {
+		t.Fatalf("expected 1 task, got %d", len(tasks))
+	}
+	if tasks[0].DueDate == nil {
+		t.Fatal("expected DueDate to be set")
+	}
+	if y, m, d := tasks[0].DueDate.Date(); y != 2026 || m != time.July || d != 29 {
+		t.Errorf("expected due date 2026-07-29, got %04d-%02d-%02d (task shifted a day earlier than Google reported)", y, m, d)
+	}
+}