#!/usr/bin/perl -p

use Getopt::Std;

BEGIN {
	getopts("f:l");
	if ($opt_f) {
		$pre = "^".('[^\t]*\t'x($opt_f-1));
	} else {
		$pre = '';
	}
}

s/($pre)(\d{9}\d*)/$1.convert($2)/geo;

sub convert {
	($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($_[0]);
	$year += 1900; $mon++;
	return sprintf "%04d.%02d.%02d %02d:%02d:%02d", $year, $mon, $mday, $hour, $min, $sec;
}