Código PHP:
$pago = mktime(0,0,0,$mes_pago,$dia_pago,$anio_pago);
$hoy = mktime(0,0,0, date("m"),date("d"), date("Y"));
$vence = mktime(0,0,0,$mes_vence,$dia_vence,$anio_vence);
$total = $vence - $pago;
$parcial = $vence - $hoy;
$porcentaje = ($parcial / $total) * 100;