Question

I’m reading a binary file using perl. In the file’s headers, there’s 4 bytes that represents MS-DOS time. How do I read this time? I’m not familiar with this format.

I’ve found this for reference: http://www.vsft.com/hal/dostime.htm but I’m still not sure how to read it.

Answer 1

Another approach:

sub mst {
  my $msdos_time = shift;
  my @t = map { ord }
          map { pack("b*", $_) }
          map { reverse($_) }
          unpack("A5 A6 A5 A5 A4 A7", unpack("b*", $msdos_time));
  my %d;
  @d{seconds,minutes,hours,day,month,year} = @t;
  $d{seconds} *= 2;
  $d{year} += 1980;
  return \%d;
}

This will work if $msdos_time is represented in little-endian format which (I believe) is how it would be laid out in memory.

(Clearly the chained map-s could be coalesced – I wrote it this way to make it easier to see what was going on.)

Example:

print Dumper(mst("\x22\x12\x01\x41"));

# byte 0   | byte 1   | byte 2   | byte 3
# 76543210 | 76543210 | 76543210 | 76543210
#    S...s                                   seconds
# ..m             M..                        minutes
#            H...h                           hours
#                          D...d             day
#                       ..m               M  month
#                                  Y.....y   year
# 00100010 | 00010010 | 00000001 | 01000001

$VAR1 = {
      'seconds' => 4,
      'hours' => 2,
      'month' => 8,
      'day' => 1,
      'minutes' => 17,
      'year' => 2012
    };