Timestamps

A handy reference to some of the more common (and some less common) ways to represent timestamps that aren't just a variation on a normal human-readable datetime string.

Contents

• Unix Epoch
• Windows FILETIME
• MS-DOS DateTime / FAT DateTime
• WebKit / Chrome
• Apple CFAbsoluteTime
• Apple HFS+
• Garmin FIT
• NTP / SNTP

Unix Epoch¹

• Number of seconds elapsed since 1970-01-01 00:00:00Z
• Typically expressed as an integer, but also sometimes as a floating point value
• Sub-second representations are also common

Examples and common variations

Value	Datetime	Accuracy
1739442600	2025-02-13 10:30:00Z	Seconds
1739442600000	2025-02-13 10:30:00Z	Milliseconds
1739442600000000	2025-02-13 10:30:00Z	Microseconds
1739442600000000000	2025-02-13 10:30:00Z	Nanoseconds
1739442600.000	2025-02-13 10:30:00Z	Seconds as floating point with millisecond precision

Windows FILETIME / Active Directory Datetime²

• Number of 100-nanosecond intervals elapsed since 1601-01-01 00:00:00Z

Examples

Value	Datetime
132223104000000000	2020-01-01 00:00:00Z
133839460990000000	2025-02-13 18:48:19Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

'''
Conversion to/from Unix Epoch seconds may be subject to rounding errors
'''

def filetime2unix(value):
    return datetime(1601, 1, 1, tzinfo=timezone.utc).timestamp() + (value / 10000000)

def unix2filetime(value):
    return int((value - datetime(1601, 1, 1, tzinfo=timezone.utc).timestamp()) * 10000000)

MS-DOS Datetime / FAT Datetime³

• A packed 32-bit value with 2-second accuracy⁴
• Year is given as an offset from 1980
• Pay attention to byte order when working with these
• ZIP file headers use MS-DOS format to encode the file last modified timestamp⁵

Structure

Bits	Description
0-4	Seconds (divided by 2)
5-10	Minute (0-59)
11-15	Hour (0-23)
16-20	Day (1-31)
21-24	Month (January == 1, February == 2, etc.)
25-31	Year (offset from 1980)

Visual Structure

      0                 1                 2                 3
      0 1 2 3 4 5 6 7   0 1 2 3 4 5 6 7   0 1 2 3 4 5 6 7   0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
     |s|s|s|s|s|m|m|m| |m|m|m|h|h|h|h|h| |D|D|D|D|D|M|M|M| |M|Y|Y|Y|Y|Y|Y|Y|
     +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
     \________/\____________/\_________/ \________/\_________/\____________/
      second      minute       hour        day       month        year

Examples

Raw Bytes (Little Endian)	Value	Datetime
\xf4\x58\x83\x59	1501780212	2024-12-03 11:07:40
\xd8\x54\x4d\x5a	1515017432	2025-02-13 10:38:48
\xdc\x54\x4d\x5a	1515017436	2025-02-13 10:38:56

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

def dos2unix(value):
    year = ((value >> 25) & 0x7F) + 1980
    month = (value >> 21) & 0x0F
    day = (value >> 16) & 0x1F
    hour = (value >> 11) & 0x1F
    minute = (value >> 5) & 0x3F
    second = (value & 0x1F) << 1

    return int(datetime(year, month, day, hour, minute, second, tzinfo=timezone.utc).timestamp())

def unix2dos(value):
    dtg = datetime.fromtimestamp(value, tz=timezone.utc)
    return ((dtg.year - 1980) << 25
            | dtg.month << 21
            | dtg.day << 16
            | dtg.hour << 11
            | dtg.minute << 5
            | dtg.hour << 11
            | dtg.minute << 5
            | dtg.second >> 1)

WebKit / Chrome⁶

• Number of microseconds elapsed since 1601-01-01 00:00:00Z
• Very similar to Windows FILETIME, but only accurate to 1000 nanoseconds (i.e. 1 microsecond) compared to Windows FILETIME's 100 nanosecond accuracy

Examples

Value	Datetime
13222310400000000	2020-01-01 00:00:00Z
13383946099000000	2025-02-13 18:48:19Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

'''
Conversion to/from Unix Epoch seconds may be subject to rounding errors
'''

def webkit2unix(value):
    return datetime(1601, 1, 1, tzinfo=timezone.utc).timestamp() + (value / 1000000)

def unix2webkit(value):
    return int((value - datetime(1601, 1, 1, tzinfo=timezone.utc).timestamp()) * 1000000)

Apple CFAbsoluteTime⁷

• Number of seconds elapsed since 2001-01-01 00:00:00Z
• Typically expressed as a floating point value with microsecond precision

Examples

Value	Datetime
729637877.045605	2024-02-14 21:11:17Z
753421722.968995	2024-11-16 03:48:42Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

def cfabsolutetime2unix(value):
    return datetime(2001, 1, 1, tzinfo=timezone.utc).timestamp() + value

def unix2cfabsolutetime(value):
    return value - datetime(2001, 1, 1, tzinfo=timezone.utc).timestamp()

Apple HFS+⁸

• Number of seconds elapsed since 1904-01-01 00:00:00Z
• Only capable of representing values between 1904-01-01 00:00:00Z and 2040-02-06 06:28:15Z
• Typically expressed as an integer
• Values do not account for leap seconds, but do include a leap day in every year that is evenly divisible by four

Examples

Value	Datetime
3203381950	2005-07-05 04:19:10Z
4294967295	2040-02-06 06:28:15Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

def hfsplus2unix(value):
    return datetime(1904, 1, 1, tzinfo=timezone.utc).timestamp() + value

def unix2hfsplus(value):
    return value - datetime(1904, 1, 1, tzinfo=timezone.utc).timestamp()

Garmin FIT⁹

• Number of seconds elapsed since 1989-12-31 00:00:00Z
• Typically expressed as an integer

Examples

Value	Datetime
938622559	2019-09-28 16:29:19Z
987654321	2021-04-18 04:25:21Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

def garmin2unix(value):
    return datetime(1989, 12, 31, tzinfo=timezone.utc).timestamp() + value

def unix2garmin(value):
    return value - datetime(1989, 12, 31, tzinfo=timezone.utc).timestamp()

NTP / SNTP ¹⁰

• A fixed point 64-bit value where the most significant 32 bits represent the number of seconds elapsed since 1900-01-01 00:00:00Z, and the least significant 32 bits represent the fractional component of the second (approx. 0.2 nanosecond precision¹¹)

Examples

Value	Datetime
14195914145254784551	2004-09-27 03:17:07.694744Z
14195914386443616565	2004-09-27 03:18:03.850895Z

Conversion to/from Unix Epoch seconds

from datetime import datetime, timezone

'''
Conversion to/from Unix Epoch seconds may be subject to rounding errors
'''

def ntp2unix(value):
    secs = value >> 32
    frac = value & 0xFFFFFFFF
    frac /= (1<<32)
    return datetime(1900, 1, 1, tzinfo=timezone.utc).timestamp() + secs + frac

def unix2ntp(value):
    secs = int(int(value) - datetime(1900, 1, 1, tzinfo=timezone.utc).timestamp())
    frac = value - int(value)
    frac *= (1<<32)
    return secs << 32 | int(frac)

Footnotes

1. https://en.wikipedia.org/wiki/Unix_time ↩

2. https://learn.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-filetime ↩

3. https://learn.microsoft.com/en-us/windows/win32/sysinfo/ms-dos-date-and-time ↩

4. https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-dosdatetimetofiletime ↩

5. https://pkwaredownloads.blob.core.windows.net/pem/APPNOTE.txt ↩

6. https://github.com/chromium/chromium/blob/43c03c93ed7c80a400ebd3be9823681f306046ce/base/time/time.h#L7 ↩

7. https://developer.apple.com/documentation/corefoundation/cfabsolutetime ↩

8. https://developer.apple.com/library/archive/technotes/tn/tn1150.html#HFSPlusDates ↩

9. https://developer.garmin.com/fit/cookbook/datetime/ ↩

10. https://datatracker.ietf.org/doc/html/rfc2030#section-3 ↩

11. https://tickelton.gitlab.io/articles/ntp-timestamps/ ↩