chronos has two statuses.

chronos_mutable_immutable.php

<?php
require 'vendor/autoload.php';

use Cake\Chronos\Chronos;

printf("Now: %s" . PHP_EOL, Chronos::now());

use Cake\Chronos\Date;

$today = new Date();
echo $today . PHP_EOL;
echo $today->modify('+3 hours') . PHP_EOL;
echo $today->modify('+3 days') . PHP_EOL;
echo $today->modify('+3 weeks') . PHP_EOL;
echo $today->modify('+3 months') . PHP_EOL;
echo $today . PHP_EOL;

echo "-- to Mutable --" . PHP_EOL;

$today2 = $today->toMutable();
echo $today2 . PHP_EOL;
echo $today2->modify('+3 hours') . PHP_EOL;
echo $today2->modify('+3 days') . PHP_EOL;
echo $today2->modify('+3 weeks') . PHP_EOL;
echo $today2->modify('+3 months') . PHP_EOL;
echo $today2 . PHP_EOL;

echo "-- to Immutable --" . PHP_EOL;

$today3 = $today2->toImmutable();
echo $today3 . PHP_EOL;
echo $today3->modify('+3 hours') . PHP_EOL;
echo $today3->modify('+3 days') . PHP_EOL;
echo $today3->modify('+3 weeks') . PHP_EOL;
echo $today3->modify('+3 months') . PHP_EOL;
echo $today3 . PHP_EOL;