Si desea ejecutar una query
con un rango de fechas puedes usar BETWEEN
declaración.
$sql = $conn->prepare("SELECT DATE(start) AS date, COUNT(id_anillos) AS total_anillos, SUM(ventas) AS total_diario_ganado
FROM PRODUCTOS WHERE start BETWEEN 'Older Date Here' AND 'Newer Date Here' ORDER BY start ASC");