Call to a member function between() on string

i have this code in my controller:

 //year 2016
$from2016 = date('2016-04-06');
$to2016 =  date('2017-04-05');

 $assets = Asset::all();
 foreach($assets as $asset)

 {

    if ($asset->purchase_date->between( $from2016,$to2016) )
    {
     $now = Carbon::now();
     if ($now->between( $from2018,$to2018) )
     {
         $asset->year= 2;
         $asset->save(); 
     }
     else if ($now->between( $from2019,$to2019) )
     {
         $asset->year=3; 
         $asset->save(); 
     }
     else if ($now->between( $from2020,$to2020) )
     {
         $asset->year=4; 
         $asset->save(); 
     }
     else if ($now->between( $from2021,$to2021) )
     {
         $asset->year=5; 
         $asset->save(); 
     }



    }

    }

but when i run it, i get the following error "Call to a member function between() on string. Anybody with an idea why i could be getting this

Thanks

via Chebli Mohamed