HP-UX

uptime | \
perl -ne '/.*up (\d+) days, +(\d+):(\d+),.*/; $total=((($1*24+$2)*60+$3)*60);
$now=time(); $now-=$total; $now=localtime($now); print $now,"\n";'

date -d @$( awk "{print \"$(date +%s)-$1\"}" /proc/uptime )

$ uptime
  9:14am  up 156 days, 20:52 ...
$ uptime | \
perl -ne '/.*up (\d+) days, +(\d+):(\d+),.*/; $total=((($1*24+$2)*60+$3)*60);
$now=time(); $now-=$total; $now=localtime($now); print $now,"\n";'
Tue Nov 11 11:21:43 2008

uptime | \
perl -ne '/.*up +(?:(\d+) days?,? +)?(\d+):(\d+),.*/; $total=((($1*24+$2)*60+$3)*60);
$now=time(); $now-=$total; $now=localtime($now); print $now,"\n";'