Description:
------------
date('Y-m-d H:i:s',strtotime('1986-05-04 00:00:00')) resutl equl 1986-05-04 01:00:00
$arr = array(
'1986-05-04 00:00:00',
'1987-04-12 00:00:00',
'1988-04-10 00:00:00',
'1989-04-16 00:00:00',
'1990-04-15 00:00:00',
'1991-04-14 00:00:00'
);
just 6 day
Datetime is same as this

Test script:
---------------
$arr = array(
'1986-05-04 00:00:00',
'1987-04-12 00:00:00',
'1988-04-10 00:00:00',
'1989-04-16 00:00:00',
'1990-04-15 00:00:00',
'1991-04-14 00:00:00'
);
foreach($arr as $date){
    echo date('Y-m-d H:i:s',strtotime($date));
}
/**
'1986-05-04 01:00:00',
'1987-04-12 01:00:00',
'1988-04-10 01:00:00',
'1989-04-16 01:00:00',
'1990-04-15 01:00:00',
'1991-04-14 01:00:00'
**/