Latest possible FILETIME

Question

I'm creating a mock object to test my application so that it works at the boundary conditions of time. I'm using FILETIME in the Windows SDK.

The link shows the earliest time which is January 1, 1601 (I'm assuming midnight 00:00:00 and both dwLowDateTime and dwHighDateTime are 0x00000000), so I have that. What is the latest possible FILETIME?

My first instinct is to set dwLowDateTime and dwHighDateTime to 0xFFFFFFFF, but then I questioned if that really is a valid time that I need to test, due to where my linked page says the SetFileTime function uses 0xFFFFFFFF to specify that a file's previous access time should be preserved.

Answer 1

My understanding is that FILETIME was made to represent any valid SYSTEMTIME in 64 bits. If you take the limit of SYSTEMTIME (last millisecond in 30827) then you end up with a FILETIME of 0x7fff35f4f06c58f0 by using SystemTimeToFileTime().

However, if you put 0x7fffffffffffffff into FileTimeToSystemTime() then you will end up in the year 30828, although this date is invalid for SYSTEMTIME. Any larger value (0x8000000000000000 and above) causes FileTimeToSystemTime() to fail.

All in all, I would recommend not to go beyond 0x7fff35f4f06c58f0 in order to stay compatible with SYSTEMTIME.

Answer 2

According to the link, FILETIME represents:

...the number of 100-nanosecond intervals since January 1, 1601 (UTC).

so not Jan 1st 1970.

It also says

...the SetFileTime function [for example] uses 0xFFFFFFFF to specify that a file's previous access time should be preserved.

So I don't think you would expect 0xFFFFFFFF to be a valid max value.

According to patent 6853957, the range is 30,000 years before/after the epoch (Jan 1, 1601). That implies you can use it with negative dates (i.e. dates before the epoch) too.

EDIT: Just calculated: it can store (approx) 58,454 days worth of 100-nanosecond intervals, so +/- 30,000 years sounds like a good value to go with, if you accept negative dates of course.

Answer 3

There is an answer in this MSDN article - Test Cases for the RTC Real-Time Functions Test:

The test looks for the range beginning with the minimum possible FILETIME (FILETIME 0 is the start of Jan 1st 1601) and end with maximum possible FILETIME (Max FILETIME is the maximum 64-bit value).