The time is still showing up with 12 hour times?
The capital G specifies a 24 hour time, so it should be showing up correctly now.
As for the date issue, just switch the m and d like this:
$mailHeader .= "Date: ". date('d-m-Y H:i:s (T)').$lineEnd;
I also changed the capital G to a capital H which is also another setting for the military time.