>>232
改良してまとめた

while read dt tm user; do
printf "%s %015d %s\n" $user `date +%s -d "$dt $tm"` $user;
done |sort -nr |uniq -2 |while read user sec user1; do
echo `date "+%m/%d/%Y %T" -d "1970-01-01 00:00:00 UTC $sec sec"` $user;
done;