Fix starting time for "show previous" links when hours is set to something

other than 12

webtools/tinderbox/showbuilds.cgi

@@ -421,7 +421,7 @@ sub print_table_header {
 sub print_table_footer {
   print "</table>\n";
 
-  my $nextdate = $maxdate - 12*60*60;
+  my $nextdate = $maxdate - $hours*60*60;
   print &open_showbuilds_href(maxdate=>"$nextdate", nocrap=>'1')
        ."Show previous $hours hours</a><br>";
   if ($hours != 24) {