It'll be your data. $row['date_and_time_entered'] has hit a condition in which it isn't parsable by the strtotime, so it instead returns false (or -1 depending on PHP version). In either case, worst case scenario you are behind by one second and that would become December 31, 1969 11:59:59 instead.
As for fixing it, it depends on what you want to do. You can easily determine if the DateTime is valid by blocking it in an if:
$iTime = strtotime($row['date_and_time_entered']);
if (version_compare(PHP_VERSION, '5.1.0', '<') && $iTime == -1 || $iTime === false)
// this is an error condition.
// this is aok
Or by using the DateTime object.
Doesn't really "fix" the problem, but at least it lets you decide what you want to do with it.