Find the difference between "today" and "today 15:00:00-05:00" and add it to "this thursday"
$sometime = "today 15:00:00-05:00";
$time1 = strtotime($sometime);
// now find only the hour part
$time = mktime(0,0,0, date('n', $time1), date('j', $time1), date('Y', $time1));
$time_difference = $time1 - $time;
$time2 = strtotime("this thursday") + $time_difference;
Or even simpler:
$sometime = "today 15:00:00-05:00";
$time1 = strtotime($sometime);
// now find only the hour part
$hour_string = date('h:i:s', $time1);
$time2 = strtotime("this thursday $hour_string");