Thanks for your idea. But let me ask you what is $off_h here? And I found that correct result occurs when I subtract $gmt[2] from $local[2]:

$diff=$local[2]-$gmt[2] + ($gmt[5] <=> $local[5] 
                                   || 
                           $gmt[7] <=> $local[7])*24;
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--dda