It should be something like this:
$filedate = date you read from flat file
//seperate the pieces of the date
list($datearray[0],$datearray[1],$datearray[2],$datearray[3],$datearray[4]) = explode(":",$filedate);
$datearray[2]+=12;
if ($datearray[2] > 24)
{
$datearray[1] += 1;
$datearray[2]=$datearray[2]-24;
}
implode(":",$datearray);
if ($filedate >= date("M:d:h:i:s"))
{
//12 hours has passed
}
this SHOULD work. I've never really tried it before.. so it's a guess...give it a try and let me know.