Laravel Eloquent How Can I Select Using Condition "where" for pivot table

I have three database tables called user(id,name), group(id,name) and user_group(user_id, group_id,valid_before) with relations many to many.

class User extends Model
{
    protected $table = 'user';

    public function groups()
    {
        return $this->belongsToMany(Group::class, 'user_group')
               ->withPivot('valid_before');
    } 
}

class Group extends Model
{
    protected $table = 'group';

    public $timestamps = false;

public function user()
{
    return $this->belongsToMany(User::class, 'user_group');
}
}

How can I select all users (using Eloquent) who have

valid_before < $some_date

?

via Chebli Mohamed