[OpenAFS-win32-devel] Important Notice: Change in Daylight Savings Time behavior for OpenAFS for Windows 1.3.81

[Jeffrey Altman]

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Starting in 1.3.81, the method by which the OpenAFS for Windows client
reports timestamp information to Windows is changing.  AFS and NTFS
both store timestamps according to UTC and not the local time zone of
the machine.  This is important because it means that time stamps have
a globally uniform time zone which can then be translated to the local
machine's time zone as needed.  The only difference between the UnixTime
used by AFS and FILETIME used by NTFS is the origination point and the
precision.  Therefore, converting between the two should be a simple
arithmetic computation.

In the past, the AFS Windows clients have used a complex algorithm by
which the client determined the local offset from UTC every five minutes
and then used that to convert the AFS time to the Windows time.  This
algorithm had several negative consequences.

(1) when the automatic DST switch occurs there is a five minute window
    during which the time zone information is considered wrong.

(2) the algorithm would adjust the UTC time in order to preserve the
    local representation of the clock.  If a file was last written at
    11:00am EST in January, in June it would report the file as having
    been written at 11:00am EDT.  Effectively, this meant that the
    time stamp on the file was converted from 16:00 UTC to 15:00 UTC
    during the summer months.  The reverse behavior would occur when
    viewing files updated during the summer and then viewed in the
    winter.

(3) because the timestamps on files stored in AFS would change each
    Spring and Fall, programs which rely on timestamps to perform
    file synchronization between AFS and other file systems would
    believe that the files had changed when in fact they had not.

The choice of how timestamps are converted to local time when DST
is active must be left up to the operating system and/or the run time
libraries.  Otherwise, there will be local inconsistencies which
will have adverse effects on applications.  Windows consistently reports
local file times relative to the offset from UTC.  Files in AFS starting
with 1.3.81 will be consistent with the Windows model.

Jeffrey Altman

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