$myTimestamp = mktime(0,0,0, 1, 23, 2003);
$myTimestamp += (180*24*3600); // add 180 days (or 180*24*3600 seconds)
now you need to find something to extract the days, the month and the year from "2003-1-23"
I don't know anything easy except using substr, I let you sort this out