Another way of adding 8 hours you could use:
$date = date('h:i:s A', time()+28800);
(28800 = seconds) Which = 8 hours
(60 x 60 x 8)
Your probably getting 0000-00-00 00:00:00 as your calculation is adding to the whole datetime so it would end up becoming an invalid calculation. I never done this before, So would need to write the code myself when I am probably at computer.