<?php
$d = new DateTime("first day of january 2017");
echo "first day of: ", $d->setDate(2015, 12, 31)->format("Y-m-d");
$d = new DateTime("last day of december 2017");
echo "\nlast day of: ", $d->setDate(2016, 1, 1)->format("Y-m-d");
Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
first day of: 2015-12-31
last day of: 2016-01-01