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);
}