<?php

class Purchase extends GraphQLType
{
    protected $attributes = [
        'name' => 'Purchase',
        'model' => PurchaseModel::class,
    ];

    public function fields(): array
    {
        return [
            // ...

            'createdAt' => new Fields\FormattableDate([
                'alias' => 'created_at',
            ]),
        ];
    }
}

<?php

class FormattableDate extends Field
{
    protected $attributes = [
        'name' => 'FormattableDate',
    ];

    public function __construct(array $settings = [])
    {
        $this->attributes = \array_merge($this->attributes, $settings);
    }

    protected function getProperty(): string
    {
        return $this->attributes['alias'] ?? $this->attributes['name'];
    }

    public function args(): array
    {
        return [
            'human' => [
                'type' => Type::boolean(),
                'defaultValue' => false,
            ],
            'format' => [
                'type' => Type::string(),
                'defaultValue' => 'Y-m-d H:i',
                'description' => 'Defaults to Y-m-d H:i',
            ],
            'relative' => [
                'type' => Type::boolean(),
                'defaultValue' => false,
            ],
        ];
    }

    public function type(): Type
    {
        return GraphQL::type('FormattableDate');
    }

    public function resolve(Model $root, array $args): string
    {
        /** @var Carbon $date */
        $date = $root->{$this->getProperty()};

        if ($args['human']) {
            return $date->format('l j F Y');
        }

        if ($args['relative']) {
            return $date->diffForHumans();
        }
        
        return $date->format($args['format']);
    }
}