Timestamps and Attributes

This document specifies the format of timestamps and file attributes in 7z archives.

Timestamps

FILETIME Format

7z uses Windows FILETIME format for all timestamps:

Size: 64 bits (8 bytes)
Unit: 100-nanosecond intervals
Epoch: January 1, 1601 00:00:00 UTC
Byte order: Little-endian

Time Properties

Property ID	Name	Description
0x12	CTime	Creation time
0x13	ATime	Last access time
0x14	MTime	Last modification time

Epoch Conversion

FILETIME to Unix timestamp:

UNIX_EPOCH_FILETIME = 116444736000000000  # 1970-01-01 in FILETIME
unix_seconds = (filetime - UNIX_EPOCH_FILETIME) / 10_000_000
unix_nanos = ((filetime - UNIX_EPOCH_FILETIME) % 10_000_000) * 100

Unix timestamp to FILETIME:

filetime = unix_seconds * 10_000_000 + UNIX_EPOCH_FILETIME
filetime += unix_nanos / 100

Value Ranges

Description	FILETIME Value
Minimum	0 (1601-01-01)
Unix epoch	116444736000000000 (1970-01-01)
Year 2000	125911584000000000
Year 2038	137919572000000000
Maximum	2^63 - 1 (year ~30828)

Note: While FILETIME is an unsigned 64-bit integer (full range 0 to 2^64 - 1), 7-Zip and most implementations treat values >= 2^63 as invalid or interpret them as signed negative values. Writers MUST NOT produce values >= 2^63. Readers encountering such values SHOULD treat them as undefined timestamps.

Example Values

# 2024-01-15 12:00:00 UTC
# Unix: 1705320000
# FILETIME: 1705320000 * 10000000 + 116444736000000000
#         = 133497936000000000
#         = 0x01DA47AA5D872000

Bytes: 00 20 87 5D AA 47 DA 01

Missing Timestamps

When a timestamp is not defined:

BooleanList indicates absence
Implementations SHOULD use a sensible default:
- Current time for creation
- File's actual mtime for modification

Unencodable Timestamps

Some timestamps cannot be represented in FILETIME format:

Pre-1601 dates: Dates before January 1, 1601 cannot be encoded. Writers SHOULD either use FILETIME value 0 or mark the timestamp as undefined in the BooleanList.
Unknown timestamps: When a file's timestamp is unknown or unavailable, mark it as undefined in the BooleanList rather than storing an arbitrary value.

File Attributes

Storage Format

Attributes are stored as UINT32 (4 bytes, little-endian):

┌────────────────────────────────┐
│ Bits 31-16 │    Bits 15-0     │
│ Unix mode  │ Windows attrs    │
└────────────────────────────────┘

Windows Attributes (Bits 0-15)

Bit	Value	Constant	Description
0	0x0001	READONLY	Read-only file
1	0x0002	HIDDEN	Hidden file
2	0x0004	SYSTEM	System file
3	-	Reserved	-
4	0x0010	DIRECTORY	Directory
5	0x0020	ARCHIVE	Archive flag
6	0x0040	DEVICE	Device (reserved)
7	0x0080	NORMAL	Normal file (no other attrs)
8	0x0100	TEMPORARY	Temporary file
9	0x0200	SPARSE_FILE	Sparse file
10	0x0400	REPARSE_POINT	Symbolic link/junction
11	0x0800	COMPRESSED	NTFS compressed
12	0x1000	OFFLINE	Offline storage
13	0x2000	NOT_CONTENT_INDEXED	Not indexed
14	0x4000	ENCRYPTED	EFS encrypted
15	0x8000	UNIX_EXTENSION	Unix mode in high bits

Unix Mode (Bits 16-31)

When bit 15 (UNIX_EXTENSION) is set, bits 16-31 contain the Unix mode:

Bits 16-18: Other permissions (rwx)
Bits 19-21: Group permissions (rwx)
Bits 22-24: Owner permissions (rwx)
Bit 25:     Sticky bit
Bit 26:     Set GID
Bit 27:     Set UID
Bits 28-31: File type

Unix Permission Bits

Bit	Mask	Description
16	0x0001 << 16	Other execute
17	0x0002 << 16	Other write
18	0x0004 << 16	Other read
19	0x0008 << 16	Group execute
20	0x0010 << 16	Group write
21	0x0020 << 16	Group read
22	0x0040 << 16	Owner execute
23	0x0080 << 16	Owner write
24	0x0100 << 16	Owner read
25	0x0200 << 16	Sticky (S_ISVTX)
26	0x0400 << 16	Set GID (S_ISGID)
27	0x0800 << 16	Set UID (S_ISUID)

Unix File Types (Bits 28-31)

Value	Mask	Type
0x1	S_IFIFO	FIFO (named pipe)
0x2	S_IFCHR	Character device
0x4	S_IFDIR	Directory
0x6	S_IFBLK	Block device
0x8	S_IFREG	Regular file
0xA	S_IFLNK	Symbolic link
0xC	S_IFSOCK	Socket

Attribute Extraction

function parse_attributes(attr: u32):
    windows_attrs = attr & 0xFFFF
    has_unix = (attr & 0x8000) != 0

    if has_unix:
        unix_mode = (attr >> 16) & 0xFFFF
        unix_type = (attr >> 28) & 0x0F
        unix_perms = (attr >> 16) & 0x0FFF
    else:
        unix_mode = None

    return (windows_attrs, unix_mode)

Symbolic Links

Storage

Symbolic links are stored as:

File with data (target path)
Special attributes indicating symlink type

Unix Symbolic Links

Attributes: UNIX_EXTENSION | (S_IFLNK << 16) | permissions

Example: 0x8000 | 0xA0000000 | 0x01FF0000 = 0xA1FF8000

Data content: UTF-8 encoded target path (no null terminator)

Windows Reparse Points

Attributes: REPARSE_POINT (0x0400)

Data content: Target path in UTF-8

Target Path Format

Relative paths: ../other/file.txt
Absolute paths: /usr/local/bin/tool (Unix) or C:\Windows\... (Windows)

Directory Handling

Detection

A directory is identified by:

EmptyStream = true AND EmptyFile = false
OR DIRECTORY attribute (0x0010) set

Attributes

Directories typically have:

Windows: DIRECTORY (0x0010)
Unix: S_IFDIR (0x4) in type bits + permissions

Path Format

Directory paths SHOULD end with /:

src/ - directory
src - could be file or directory (check attributes)

Special Files (Unix)

Device Files

Stored with appropriate type bits:

Character device: S_IFCHR (0x2)
Block device: S_IFBLK (0x6)

Data content: Major/minor device numbers (implementation-defined)

FIFO and Sockets

Stored with type bits:

FIFO: S_IFIFO (0x1)
Socket: S_IFSOCK (0xC)

Typically stored as empty files with type attributes.

Cross-Platform Considerations

Windows → Unix

Windows Attribute	Unix Handling
READONLY	Clear write bits
HIDDEN	Prefix with `.` (convention)
DIRECTORY	Set S_IFDIR
ARCHIVE	Ignore

Unix → Windows

Unix Feature	Windows Handling
Symlink	REPARSE_POINT
Execute bit	Ignore (use extension)
Permissions	Map to READONLY if no write
Device files	Cannot represent

Example Attributes

Regular File (644)

Unix: rw-r--r-- (0644)
Attribute: 0x81A48020
  Windows: 0x0020 (ARCHIVE)
  Unix ext: 0x8000
  Mode: 0x81A4 << 16
    Type: 0x8 (S_IFREG)
    Perms: 0x1A4 (rw-r--r--)

Executable (755)

Unix: rwxr-xr-x (0755)
Attribute: 0x81ED8020
  Windows: 0x0020 (ARCHIVE)
  Unix ext: 0x8000
  Mode: 0x81ED << 16
    Type: 0x8 (S_IFREG)
    Perms: 0x1ED (rwxr-xr-x)

Directory (755)

Unix: drwxr-xr-x
Attribute: 0x41ED8010
  Windows: 0x0010 (DIRECTORY)
  Unix ext: 0x8000
  Mode: 0x41ED << 16
    Type: 0x4 (S_IFDIR)
    Perms: 0x1ED (rwxr-xr-x)

Symbolic Link

Unix: lrwxrwxrwx
Attribute: 0xA1FF8000
  Windows: 0x0000 (or 0x0400 for reparse)
  Unix ext: 0x8000
  Mode: 0xA1FF << 16
    Type: 0xA (S_IFLNK)
    Perms: 0x1FF (rwxrwxrwx)

Timestamps and Attributes ​

Timestamps ​

FILETIME Format ​

Time Properties ​

Epoch Conversion ​

Value Ranges ​

Example Values ​

Missing Timestamps ​

Unencodable Timestamps ​

File Attributes ​

Storage Format ​

Windows Attributes (Bits 0-15) ​

Unix Mode (Bits 16-31) ​

Unix Permission Bits ​

Unix File Types (Bits 28-31) ​

Attribute Extraction ​

Symbolic Links ​

Storage ​

Unix Symbolic Links ​

Windows Reparse Points ​

Target Path Format ​

Directory Handling ​

Detection ​

Attributes ​

Path Format ​

Special Files (Unix) ​

Device Files ​

FIFO and Sockets ​

Cross-Platform Considerations ​

Windows → Unix ​

Unix → Windows ​

Example Attributes ​

Regular File (644) ​

Executable (755) ​

Directory (755) ​

Symbolic Link ​

See Also ​