Description:
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I found out that setISODate function does not set timestamp as expected. It sets proper date but time seems to be taken from current timestamp. I know that it is meant to set date but it should set 00:00:00 for time part to ensure repeatable results.

Test script:
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$date='2018-06-29 00:00:00';
$startDate=(new DateTime())->setISODate(date('Y',strtotime($date)),date('W',strtotime($date)))->format('Y-m-d H:i:s');
$endDate=(new DateTime())->setISODate(date('Y',strtotime($date)), date('W',strtotime($date)), 7)->format('Y-m-d H:i:s');
$startTimestamp = (new DateTime())->setISODate(date('Y',strtotime($date)),date('W',strtotime($date)))->getTimestamp();
echo 'input = '.$date;
echo '<br/>strtotime = '.strtotime($date);
echo '<br/>setISODate->getTimestamp = ';
echo $startTimestamp;
echo '</br>startDate = '.$startDate.'<br/>endDate = '.$endDate;

Expected result:
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input = 2018-06-29 00:00:00
strtotime = 1530244800
setISODate->getTimestamp = 1529899200
startDate = 2018-06-25 00:00:00
endDate = 2018-07-01 00:00:00 

Actual result:
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input = 2018-06-29 00:00:00
strtotime = 1530244800
setISODate->getTimestamp = 1529946301
startDate = 2018-06-25 13:05:01
endDate = 2018-07-01 13:05:01 

or bit later:

input = 2018-06-29 00:00:00
strtotime = 1530244800
setISODate->getTimestamp = 1529946704
startDate = 2018-06-25 13:11:44
endDate = 2018-07-01 13:11:44