- strtotime: documentation ( source)
<?php
$testDates = array(
'2018-01-01',
'2000-01-01',
'1992-01-01',
'1991-12-30',
'1980-01-01',
'1971-01-01',
'1970-01-02',
'1970-01-01',
);
foreach($testDates as $testDate){
echo $testDate . ' - result:' . strtotime($testDate) . PHP_EOL;
}