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use POSIX 'strftime';

...

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

# convert data2seconds, should be good until the year 2100

use Time::Local 'timelocal_nocheck';

...

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

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

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

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

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

my %data = (
  Y => $year,
...
);

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