MS-DOS datetime: Ruby parsing library

MS-DOS date and time are packed 16-bit values that specify local date/time. The time is always stored in the current UTC time offset set on the computer which created the file. Note that the daylight saving time (DST) shifts also change the UTC time offset.

For example, if you pack two files A and B into a ZIP archive, file A last modified at 2020-03-29 00:59 UTC+00:00 (GMT) and file B at 2020-03-29 02:00 UTC+01:00 (BST), the file modification times saved in MS-DOS format in the ZIP file will vary depending on whether the computer packing the files is set to GMT or BST at the time of ZIP creation.

  • If set to GMT:
    • file A: 2020-03-29 00:59 (UTC+00:00)
    • file B: 2020-03-29 01:00 (UTC+00:00)
  • If set to BST:
    • file A: 2020-03-29 01:59 (UTC+01:00)
    • file B: 2020-03-29 02:00 (UTC+01:00)

It follows that you are unable to determine the actual last modified time of any file stored in the ZIP archive, if you don't know the locale time setting of the computer at the time it created the ZIP.

This format is used in some data formats from the MS-DOS era, for example:

KS implementation details

License: CC0-1.0
Minimal Kaitai Struct required: 0.9

References

This page hosts a formal specification of MS-DOS datetime using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Parse a local file and get structure in memory:

data = DosDatetime.from_file("path/to/local/file.MS-DOS datetime")

Or parse structure from a string of bytes:

bytes = "\x00\x01\x02..."
data = DosDatetime.new(Kaitai::Struct::Stream.new(bytes))

After that, one can get various attributes from the structure by invoking getter methods like:

data.time # => get time

Ruby source code to parse MS-DOS datetime

dos_datetime.rb

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

require 'kaitai/struct/struct'

unless Gem::Version.new(Kaitai::Struct::VERSION) >= Gem::Version.new('0.9')
  raise "Incompatible Kaitai Struct Ruby API: 0.9 or later is required, but you have #{Kaitai::Struct::VERSION}"
end


##
# MS-DOS date and time are packed 16-bit values that specify local date/time.
# The time is always stored in the current UTC time offset set on the computer
# which created the file. Note that the daylight saving time (DST) shifts
# also change the UTC time offset.
# 
# For example, if you pack two files A and B into a ZIP archive, file A last modified
# at 2020-03-29 00:59 UTC+00:00 (GMT) and file B at 2020-03-29 02:00 UTC+01:00 (BST),
# the file modification times saved in MS-DOS format in the ZIP file will vary depending
# on whether the computer packing the files is set to GMT or BST at the time of ZIP creation.
# 
#   - If set to GMT:
#       - file A: 2020-03-29 00:59 (UTC+00:00)
#       - file B: 2020-03-29 01:00 (UTC+00:00)
#   - If set to BST:
#       - file A: 2020-03-29 01:59 (UTC+01:00)
#       - file B: 2020-03-29 02:00 (UTC+01:00)
# 
# It follows that you are unable to determine the actual last modified time
# of any file stored in the ZIP archive, if you don't know the locale time
# setting of the computer at the time it created the ZIP.
# 
# This format is used in some data formats from the MS-DOS era, for example:
# 
#   - [zip](/zip/)
#   - [rar](/rar/)
#   - [vfat](/vfat/) (FAT12)
#   - [lzh](/lzh/)
#   - [cab](http://justsolve.archiveteam.org/wiki/Cabinet)
# @see https://docs.microsoft.com/en-us/windows/win32/sysinfo/ms-dos-date-and-time Source
# @see https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-dosdatetimetofiletime Source
# @see https://github.com/reactos/reactos/blob/c6b6444/dll/win32/kernel32/client/time.c#L82-L87 DosDateTimeToFileTime
# @see https://download.microsoft.com/download/0/8/4/084c452b-b772-4fe5-89bb-a0cbf082286a/fatgen103.doc page 25/34
class DosDatetime < Kaitai::Struct::Struct
  def initialize(_io, _parent = nil, _root = self)
    super(_io, _parent, _root)
    _read
  end

  def _read
    @time = Time.new(@_io, self, @_root)
    @date = Date.new(@_io, self, @_root)
    self
  end
  class Time < Kaitai::Struct::Struct
    def initialize(_io, _parent = nil, _root = self)
      super(_io, _parent, _root)
      _read
    end

    def _read
      @second_div_2 = @_io.read_bits_int_le(5)
      raise Kaitai::Struct::ValidationGreaterThanError.new(29, second_div_2, _io, "/types/time/seq/0") if not second_div_2 <= 29
      @minute = @_io.read_bits_int_le(6)
      raise Kaitai::Struct::ValidationGreaterThanError.new(59, minute, _io, "/types/time/seq/1") if not minute <= 59
      @hour = @_io.read_bits_int_le(5)
      raise Kaitai::Struct::ValidationGreaterThanError.new(23, hour, _io, "/types/time/seq/2") if not hour <= 23
      self
    end
    def second
      return @second unless @second.nil?
      @second = (2 * second_div_2)
      @second
    end
    def padded_second
      return @padded_second unless @padded_second.nil?
      @padded_second = (second <= 9 ? "0" : "") + second.to_s(10)
      @padded_second
    end
    def padded_minute
      return @padded_minute unless @padded_minute.nil?
      @padded_minute = (minute <= 9 ? "0" : "") + minute.to_s(10)
      @padded_minute
    end
    def padded_hour
      return @padded_hour unless @padded_hour.nil?
      @padded_hour = (hour <= 9 ? "0" : "") + hour.to_s(10)
      @padded_hour
    end
    attr_reader :second_div_2
    attr_reader :minute
    attr_reader :hour
  end
  class Date < Kaitai::Struct::Struct
    def initialize(_io, _parent = nil, _root = self)
      super(_io, _parent, _root)
      _read
    end

    def _read
      @day = @_io.read_bits_int_le(5)
      raise Kaitai::Struct::ValidationLessThanError.new(1, day, _io, "/types/date/seq/0") if not day >= 1
      @month = @_io.read_bits_int_le(4)
      raise Kaitai::Struct::ValidationLessThanError.new(1, month, _io, "/types/date/seq/1") if not month >= 1
      raise Kaitai::Struct::ValidationGreaterThanError.new(12, month, _io, "/types/date/seq/1") if not month <= 12
      @year_minus_1980 = @_io.read_bits_int_le(7)
      self
    end

    ##
    # only years from 1980 to 2107 (1980 + 127) can be represented
    def year
      return @year unless @year.nil?
      @year = (1980 + year_minus_1980)
      @year
    end
    def padded_day
      return @padded_day unless @padded_day.nil?
      @padded_day = (day <= 9 ? "0" : "") + day.to_s(10)
      @padded_day
    end
    def padded_month
      return @padded_month unless @padded_month.nil?
      @padded_month = (month <= 9 ? "0" : "") + month.to_s(10)
      @padded_month
    end
    def padded_year
      return @padded_year unless @padded_year.nil?
      @padded_year = (year <= 999 ? "0" + (year <= 99 ? "0" + (year <= 9 ? "0" : "") : "") : "") + year.to_s(10)
      @padded_year
    end
    attr_reader :day
    attr_reader :month
    attr_reader :year_minus_1980
  end
  attr_reader :time
  attr_reader :date
end