Lets say you have lost your wtmp file or its corrupted. Then you can use below script to find the last reboot time of any UNIX flavour.
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";'