Description:
------------
strftime(%G) for Jan 1st 2017 returns 2016 instead of 2017. 

trying with other years works as expected (2013..2015 and 2018..2020) but others do not (2005, 2006, 2010..2012, 2016, 2017). these lists are not exhaustive.

tested on 7.0.10 and 5.6.7 php versions.

the date function works as expected. 

here's a second 19-line script that looks for the first January day where %G works as expected:

-----------------
<?php

echo "php version: " . phpversion() . "\n";

for ($year=1970;$year<2030;$year++) {

	$day = 1;
	$seconds = mktime(0,0,0,1,$day,$year);
	while ( strftime("%G",$seconds) != $year ) {
		$day ++;
		$seconds = mktime(0,0,0,1,$day,$year);
	}

	if ($day == 1)
		echo "$year: ok\n";
	else
		echo "$year: it is not $year until " . strftime("%c",$seconds) . "\n";
}
----------------------------------

thanks.


Test script:
---------------
<?php

echo "php version: " . phpversion() . "\n";

$seconds = mktime(0,0,0,1,1,2017);
echo "seconds from epoch: $seconds\n";
echo "strftime(%G): " . strftime("%G",$seconds) . "\n";
echo "strftime(%c): " . strftime("%c",$seconds) . "\n";
echo "date(Y)     : " . date("Y",$seconds)      . "\n";
?>


Expected result:
----------------
php version: 7.0.10-1
seconds from epoch: 1483225200
strftime(%G): 2017
strftime(%c): Sun Jan  1 00:00:00 2017
date(Y)     : 2017

Actual result:
--------------
php version: 7.0.10-1
seconds from epoch: 1483225200
strftime(%G): 2016
strftime(%c): Sun Jan  1 00:00:00 2017
date(Y)     : 2017