Solution For Rsync and Cygwin Daylight Savings Timezone Problems

Wayne Piekarski wayne at
Wed Apr 2 21:25:10 EST 2003

> Thanks for the reminder.  Unfortunately your email was
> rambling so that it was unclear what can actually be done to
> avoid the problem.  Here in the US Daylight savings time
> will take effect this coming Sunday.

Sorry about the rambling :) I wanted to dump out everything I'd learned
because it was quite confusing initially and there was nothing available on
how to fix it.

The hack I made corrects the problem as such, but I'm not sure if it has
other effects that will break other applications. By disabling the daylight
savings tick box Windows will not automatically adjust its time, but who
knows what else? I wouldn't want to say that this is the correct solution
just yet. I've emailed the Cygwin mailing list with the same thing in case
someone spots something there and maybe a possible fix to make the DLLs deal
with this.

> With the transition between standard time and daylight
> savings time MS-Windows systems are known to appear to
> change the modification time of existing files.  The effect
> of this is to give the appearance that every file has
> changed.  This will cause affected transfers to take
> inordinate amounts of time.  This affects FAT filesystems
> which store times in localtime and not NTFS which uses UTC.

I have not looked into NTFS but I do know it has wierd issues with UTC as
well, probably not to the extent of FAT but I don't think it is as clean as
Unix-ish implementations.

> The impact of this may be minimized by running rsync with
> the --modify-window=3601 command-line option.  This will
> cause rsync to ignore modification time differences of one
> hour will allow rsync jobs to complete in the usual time
> period with a minimal impact on backup integrity.  To get
> back to normal it will be necessary to run rsync with the
> usual modify-window on all files.  This can be done in
> stages.

If you run with this flag, does it just skip the updates on those files, or
does it update the time stamp mtime as well? I'm thinking it just skips them
and you will have to run with --modify-window until DST ends a few months


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