Here's an example of what do to if you desire to use Laravel's updateExistingPivot() while getting your attributes mutated through a pivot model.

Employee.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;

class Employee extends Model
{
    /**
     * Return all the requirements that are assigned to the employee.
     *
     * @return BelongsToMany
     */
    public function requirements() : BelongsToMany
    {
        return $this->belongsToMany(Requirement::class)
                    ->withTimestamps()
                    ->withPivot([
                        'completion_date',
                    ])
                    ->using(EmployeeRequirement::class);
    }
}

EmployeeRequirement.php

<?php

namespace App;

use Carbon\Carbon;
use Illuminate\Database\Eloquent\Relations\Pivot;

class EmployeeRequirement extends Pivot
{    
   /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'completion_date',
    ];
  
    /**
     * Convert a provide completion date to a standard date format.
     *
     * @param $date
     *
     * @return void
     */
    public function setCompletionDateAttribute($date)
    {
        $this->attributes['completion_date'] = $date ? Carbon::parse($date)->toDateString() : null;
    }
}

Requirement.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;

class Requirement extends Model
{
  // Blah blah blah...
}

Scenarios.php

<?php

$attributes = [
  'completion_date' => '11MAY13',
];

// Obtain the target employee and requirement record.
$employee    = App\Employee::first();
$requirement = \App\Requirement::first();

// Attach the requriement to the employee. This will cause completion_date to be mutated to '2013-11-05' on insert.
$employee->requirements()->attach($requirement, $attributes);

# Now you decide you need to update the record.

// Under this scenario Laravel would attempt to inject 11MAY13 into the completion_date column which will break the database.
$employee->requirements()->updateExistingPivot($requirement->id, $attributes);

// Using this approach would allow your date to be mutated.
$employee->requirements()->updateExistingPivot($requirement->id, (new \App\EmployeeRequirement($attributes))->toArray());