Your constructor sets time in terms of localtime. Your half-hour calcuation subtracts a half hour from the object that was based on localtime. Your $oldtime object is constructed using strptime, and is not setting a timezone offset.

You can see this in action by adding the following lines:

print "\$ctime tzoffset:   ", $ctime->tzoffset, "\n";
print "\$halfago tzoffset: ", $halfago->tzoffset, "\n";
print "\$oldtime tzoffset: ", $oldtime->tzoffset, "\n";
[download]

which will produce output along the line of:

$ctime tzoffset:   -25200
$halfago tzoffset: -25200
$oldtime tzoffset: 0
[download]

(...except that the offsets will be in your own localtime, not mine.)

You might be able to set the $oldtime->tzoffset($some_value), or preferably by providing a %Z or %z field to strptime.