src/Carbon/Traits/Units.php
public function subUnit($unit, $value = 1, $overflow = null)
    {
        return $this->addUnit($unit, -$value, $overflow);
    }
public function subUnit($unit, $value = 1, $overflow = null)
    {
        return $this->addUnit($unit, -$value, $overflow);
    }
public function subUnit ( $ unit , $ value = 1 , $ overflow = null ) { return $ this -> addUnit ( $ unit , - $ value , $ overflow ) ; }
Subtract given units to the current instance.

@param string    $unit
@param int       $value
@param bool|null $overflow

@return CarbonInterface
Subtract given units to the current instance .