julienmonnerie/kirby/config/fields/mixins/datetime.php

<?php

use Kirby\Toolkit\Date;

return [
	'props' => [
		/**
		 * Defines a custom format that is used when the field is saved
		 */
		'format' => function (string|null $format = null) {
			return $format;
		}
	],
	'methods' => [
		'toDatetime' => function ($value, string $format = 'Y-m-d H:i:s') {
			if ($date = Date::optional($value)) {
				if ($this->step) {
					$step = Date::stepConfig($this->step);
					$date->round($step['unit'], $step['size']);
				}

				return $date->format($format);
			}

			return null;
		}
	],
	'save' => function ($value) {
		if ($date = Date::optional($value)) {
			return $date->format($this->format);
		}

		return '';
	},
];