if I understand right, it is a bit tricky. I would first store the time difference as an absolute value:
$yourtime = -($mtime - time()))
Then you can get the days with
$sdays = (int)($yourtime / 3600)
For the hours and minutes you could take
$timedifference = $yourtime - ((int)($yourtime / 3600) * 3600)
and format it with the date() function.
I hope, I do not have a mistake in my thinking. I did not find any character in the date() function that gives back absolute days and I do not know an easier method.
Hope it is of help.
Grius