Default datetimes in repeatable fields

I’ve got a need for a repeatable text_datetime_timestamp field, and I’d like to set the default to “today + a month”. In my code, I have:

$cmb2->add_field(
	array(
		'id' => 'my_prefix_showtime',
		'name' => __( 'Showtimes', 'my-custom-post' ),
		'type' => 'text_datetime_timestamp',
		'repeatable' => true,
		'default' => time() + MONTH_IN_SECONDS,
	)
);

This works fine for the first new date (it shows up as a formatted date a month in the future); however, any future fields get what appears to be the raw time() + MONTH_IN_SECONDS output.

Is there any way to ensure that every datetime field in the repeatable box gets a valid date?