Block diagram
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.
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:
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.
meta:
id: dos_datetime
title: MS-DOS datetime
xref:
justsolve: MS-DOS_date/time
tags:
- dos
license: CC0-1.0
ks-version: 0.9
bit-endian: le
doc: |
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)
doc-ref:
- https://learn.microsoft.com/en-us/windows/win32/sysinfo/ms-dos-date-and-time
- https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-dosdatetimetofiletime
- https://github.com/reactos/reactos/blob/c6b64448ce4/dll/win32/kernel32/client/time.c#L82-L87 DosDateTimeToFileTime
- https://download.microsoft.com/download/0/8/4/084c452b-b772-4fe5-89bb-a0cbf082286a/fatgen103.doc page 25/34
-webide-representation: "{date} {time}"
seq:
- id: time
type: time
- id: date
type: date
types:
time:
-webide-representation: "{padded_hour}:{padded_minute}:{padded_second}"
seq:
- id: second_div_2
type: b5
valid:
max: 29 # 0-58 seconds
- id: minute
type: b6
valid:
max: 59
- id: hour
type: b5
valid:
max: 23
instances:
second:
value: 2 * second_div_2
padded_second:
value: '(second <= 9 ? "0" : "") + second.to_s'
padded_minute:
value: '(minute <= 9 ? "0" : "") + minute.to_s'
padded_hour:
value: '(hour <= 9 ? "0" : "") + hour.to_s'
date:
-webide-representation: "{padded_year}-{padded_month}-{padded_day}"
seq:
- id: day
type: b5
valid:
min: 1
- id: month
type: b4
valid:
min: 1
max: 12
- id: year_minus_1980
type: b7
instances:
year:
value: 1980 + year_minus_1980
doc: only years from 1980 to 2107 (1980 + 127) can be represented
padded_day:
value: '(day <= 9 ? "0" : "") + day.to_s'
padded_month:
value: '(month <= 9 ? "0" : "") + month.to_s'
padded_year:
value: |
(year <= 999 ? "0" +
(year <= 99 ? "0" +
(year <= 9 ? "0" : "")
: "")
: "") + year.to_s