Here is my code and I do not know why the two DateTime values are quite different.
<?php
$Record['DateTime']='31-May-2018 19-15-28';
$DateTime = date('d-M-Y H-i-s', strtotime($Record['DateTime']));
$Record['Send_Time'] = date('d-M-Y H:i:s', strtotime($DateTime));
print_r($Record);
And here is my result-
Array
(
[DateTime] => 31-May-2018 19-15-28
[Send_Time] => 01-Jan-1970 01:00:00
)