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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Windows does not give your files a 1601 creation date. It stores file times as a count of 100-nanosecond intervals measured from 00:00 UTC on January 1, 1601, and that starting point is simply a reference date for the count. Once you see the count as an offset, the odd-looking start date makes sense, and the places where timestamps can mislead you become much easier to spot.
What the stored value actually is
Microsoft defines a file time in its File Times documentation as “a 64-bit value that represents the number of 100-nanosecond intervals that have elapsed since 12:00 A.M. January 1, 1601 Coordinated Universal Time (UTC).” The structure that carries it, FILETIME, is split into two 32-bit halves, dwLowDateTime and dwHighDateTime.
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Three consequences follow from that definition:
- A value of zero is the epoch itself, 1601-01-01 00:00 UTC. It is not a creation date for anything.
- Each step of the counter is one ten-millionth of a second. A 64-bit counter at that step size covers roughly 58,000 years from the epoch, so the range is not a practical limit.
- The counter is an absolute point in time, not a local clock reading. Converting it to a wall-clock date is a separate step that depends on the time zone and, for some filesystems, on how the time was stored.
The Unix epoch makes a useful comparison. Between 1601 and 1970 there are 369 years containing 89 leap years, which is 134,774 days or 11,644,473,600 seconds. In FILETIME units, 1970-01-01 00:00 UTC is therefore 116,444,736,000,000,000.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →| Moment (UTC) | FILETIME count (100-ns units) |
|---|---|
| 1601-01-01 00:00:00 | 0 |
| 1970-01-01 00:00:00 | 116,444,736,000,000,000 |
The second row is arithmetic from the definition above rather than a value quoted from a source, but it is the number you will see if you convert a Unix timestamp into a Windows one.
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Why 1601 was chosen
Raymond Chen, a long-time Microsoft engineer and author of the Old New Thing blog, gave the most detailed public explanation in a March 2009 post titled Why is the Win32 epoch January 1, 1601? His account is that the Gregorian calendar repeats on a 400-year cycle, and that:
“The Gregorian calendar operates on a 400-year cycle, and 1601 is the first year of the cycle that was active at the time Windows NT was being designed. In other words, it was chosen to make the math come out nicely.”
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Chen also says he received email from Dave Cutler confirming this explanation. That rationale is Chen’s historical account. Microsoft’s API documentation defines what FILETIME means but does not itself state why 1601 was picked.
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The practical point is that a repeating 400-year calendar makes leap-year and century rules line up neatly at the start of each cycle. Counting from the start of a full cycle keeps the arithmetic for converting counts into calendar dates simple, which is the design goal Chen describes.
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Reading a FILETIME correctly
Microsoft’s guidance is to copy the two 32-bit halves into a ULARGE_INTEGER and do 64-bit arithmetic on its QuadPart member, rather than doing arithmetic on the FILETIME structure directly. To turn a value into calendar fields, use FileTimeToSystemTime.
- Obtain the FILETIME, for example from
GetFileTimeor the file’s properties. - Copy
dwLowDateTimeintoLowPartanddwHighDateTimeintoHighPartof aULARGE_INTEGER. - To get Unix seconds, divide
QuadPartby 10,000,000 and subtract 11,644,473,600. - To get calendar fields in UTC, pass the FILETIME to
FileTimeToSystemTime. UseFileTimeToLocalFileTimefirst only if you need local time, and remember that the result depends on the time zone in effect when you convert.
ULARGE_INTEGER u;
u.LowPart = ft.dwLowDateTime;
u.HighPart = ft.dwHighDateTime;
unsigned long long unixSeconds = (u.QuadPart / 10000000ULL) - 11644473600ULL;Where the 100-nanosecond unit stops being the precision
The counter’s unit is not a promise about how precisely each filesystem records time. The two main Windows filesystems differ in how they store time and how often they update it, and Microsoft’s documentation makes those differences explicit.
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| Behaviour | FAT | NTFS |
|---|---|---|
| Time basis on disk | Local computer time | UTC |
| Creation time resolution | 10 milliseconds | Not stated in the cited Microsoft page |
| Write time resolution | 2 seconds | Not stated in the cited Microsoft page |
| Access time resolution | 1 day (access date only) | Last-access updates can be delayed by up to one hour |
The FAT figures come from Microsoft’s Win32 documentation on file times. The NTFS access-time delay is described in the same documentation. Because the FAT times are local time, an hour can appear to shift around daylight-saving-time changes. Microsoft describes this for cached FAT times, which keep the discrepancy until the machine restarts.
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Common misreadings
- “My files were created in 1601.” The count starts in 1601, but a file’s timestamp is whatever count the filesystem wrote when the file was created or changed. A zero count would decode to 1601, but that is a stored value, not an age.
- “Windows has existed since 1601.” 1601 is the reference date for the encoding, not the start of Windows or of any installation.
- “Ten-millionths of a second means the time is that precise.” The unit sets the granularity of the number. Each filesystem decides how finely it records and updates a given timestamp, as the table shows.
- “A FAT time and an NTFS time mean the same thing.” A FAT time is local time and an NTFS time is UTC, so the same wall-clock reading can correspond to different absolute moments.
If a timestamp looks wrong, check the filesystem first, then the time zone in effect when it was written, and only then the arithmetic.
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