use POSIX 'strftime';

## similar to strftime but with milliseconds via "%q"
sub sec2date {
    my ($fmt, $sec) = @_;
    
    ## fractional part of $sec, rounded to 3 digits
    my $milli = sprintf "%03.0f", 1000 * ($sec - int $sec);

    ## %q becomes our milliseconds
    $fmt =~ s/%q/$milli/g;

    strftime $fmt, localtime(int $sec);
}

##</code><code>##

# convert data2seconds, should be good until the year 2100

##</code><code>##

use Time::Local 'timelocal_nocheck';

## given year+julian date,
my $epoch = timelocal_nocheck $sec, $min, $hour, $julian, 0, $year-1900;

## or given y/m/d (could use normal "timelocal" here)
my $epoch = timelocal_nocheck $sec, $min, $hour, $day, $mon-1, $year-1900;

## add milliseconds on at the end:
$epoch .= ".$milli";

##</code><code>##

$sec  = "0" x (2 - length($time)).$time ;
$msec = "0" x (3 - length($msec)).$msec ;

##</code><code>##

$sec  = sprintf "%02d", $time;
$msec = sprintf "%03d", $msec;

##</code><code>##

($format = $format ) =~ s/\%Y/$year/g ;  # xxxx
($format = $format ) =~ s/\%m/$month/g ; # 1-12
($format = $format ) =~ s/\%d/$day/g ;   # 01-31

##</code><code>##

for ($format) {
    s/%Y/$year/g;
    s/%m/$month/g;
    ...
}

##</code><code>##

my %data = (
  Y => $year,
  m => $month,
  d => $day,
  ...
);

$format =~ s/\%([A-Za-z])/ exists $data{$1} ? $data{$1} : "%$1" /ge;