main-disk-free-convert-dates.php

<?php

$json1 = <<<JSON
...
JSON;
var_dump(strlen($json1));
$data1 = json_decode($json1, true);
// print_r($data1);

$data2 = [
	'start' => date2int(key($data1)),
	'days' => [],
];
foreach ($data1 as $date => $info) {
	$data2['days'][] = $info;
}
var_dump(strlen(json_encode($data2)));
echo "first day = " . int2date($data2['start']) . "\n";
echo "last day = " . int2date($data2['start'] + count($data2['days']) - 1) . "\n";
print_r($data2);

// ==== //

function date2int(string $date) : int {
	$components = array_map(intval(...), explode('-', $date));
	return juliantojd($components[1], $components[2], $components[0]);
}

function int2date(int $date) : string {
	return date('Y-m-d', strtotime(jdtojulian($date)));
}