I'm in the interesting position of being BOTH a Brit AND an Naturalized Colonial Upstart so I go with the recommended Universal format. This is what I have:

In w3t.pm:

###########################################################################
# convert_time: converts the time to a useable format
###########################################################################
sub convert_time {

my $time = shift;
$time = localtime($time);
if($theme{'timeformat'} =~/^short/) {

my @tarray = split(/ +/,$time);
my %months = (
"Jan" => 1,
"Feb" => 2,
"Mar" => 3,
"Apr" => 4,
"May" => 5,
"Jun" => 6,
"Jul" => 7,
"Aug" => 8,
"Sep" => 9,
"Oct" => 10,
"Nov" => 11,
"Dec" => 12
);
my $year = substr($tarray[4],2,2);
my ($hour,$min,$sec) = split(/:/,$tarray[3]);
my $AMPM;

if ($theme{'timeformat'} ne "short4") {
if ($hour >= 12) {
$AMPM = "PM";
if ($hour > 12) {
$hour = $hour-12;
if ($hour < 10) {
$hour = "0$hour";
}
}
} else {
$AMPM = "AM";
}
}

my $day = $tarray[2];
if ($day < 10) {
$day = "0$day";
}

my $month = $months{$tarray[1]};
if ($month < 10) {
$month = "0$month";
}


if ($theme{'timeformat'} eq "short1") {
# $time = "$months{$tarray[1]}/$tarray[2]/$year $hour:$min $AMPM";
$time = "$month/$day/$year $hour:$min $AMPM";
}
elsif ($theme{'timeformat'} eq "short2") {
# $time = "$tarray[2]/$months{$tarray[1]}/$year $hour:$min $AMPM";
$time = "$day/$month/$year $hour:$min $AMPM";
}
elsif ($theme{'timeformat'} eq "short3") {
# $time = "$year/$months{$tarray[1]}/$tarray[2] $hour:$min $AMPM";
$time = "$year/$month/$day $hour:$min $AMPM";
}
elsif ($theme{'timeformat'} eq "short4") {
# $time = "$year/$months{$tarray[1]}/$tarray[2] $hour:$min";
$time = "$year/$month/$day $hour:$min";
}

}
return $time;

}

In w3ttheme.pm:

# Which time format to use. The short format is Y2K compliant <img border="0" title="" alt="[Smile]" src="images/icons/smile.gif" />
# long = 'Sun Mar 7 13:19:48 1999';
# short1 = 'MM/DD/YY 01:19 PM';
# short2 = 'DD/MM/YY 01:19 PM';
# short3 = 'YY/MM/DD 01:19 PM';
# Scream's short4 was = 'DD/MM/YYYY 10:19';
# but My short4 = 'YY/MM/DD 13:19';
$theme{'timeformat'} = 'short4';


<img src="http://www.amdragon.com/images/eileensig.gif" alt=" - " />