Re: PROBLEM: dos filesystem timestamps and daylight savings time

[Jan Hudec]

On Tue, Oct 29, 2002 at 08:08:20PM -0500, Kenneth M. Howlett wrote:
> A few days ago, daylight savings time ended, and now
> ls --full-time says the timestamps of all the files on
> my dos partition have increased by one hour.
> 
> For example, ls --full-time says the timestamp of command.com is:
> last week: Tue Apr 07 06:00:00 1992
> this week: Tue Apr 07 07:00:00 1992
> 
> I think the timestamps of a dos filesystem are stored in local
> time. So the dos filesystem driver needs to convert the local
> time to unix standard time, and then ls converts back to local
> time, and displays the timestamp in local time.
> 
> I think that the problem is that the dos filesystem driver's
> local time to unix standard time algorithm is compensating for
> whether or not daylight savings time is in effect NOW. It should
> be compensating for whether or not daylight savings time was in
> effect at the time of the timestamp.
> 
> The time conversion algorithm is function date_dos2unix in file
> /usr/src/linux-2.4.19/fs/fat/misc.c. Is there a way to use
> tz_minuteswest from the the time of the timestamp instead of the
> current tz_minuteswest?
> 
> Or before returning the number of seconds, function date_dos2unix
> could determine if daylight savings time is in effect now, and if
> daylight savings time was in effect at the time of the timestamp.
> These determinations could return 0 or 1. Then subtract the two
> determinations, which will give us -1, 0, or 1. Multiply by 3600
> and add to the number of seconds.
> 
> Function fat_date_unix2dos in file
> /usr/src/linux-2.4.19/fs/fat/misc.c should have a similar fix.
> 
> ls appears to convert unix standard time to local time correctly,
> adjusting for whether or not daylight savings time was in effect
> at the time being converted. Maybe we should look at the source
> for ls, to see how ls converts time.

Yes. But it needs the /usr/share/zoneinfo/`cat /etc/timezone` file for this.
That file contains a record of when daylight saving time was in effect
for that country. It's handling is in standart C library. But kernel
does not use C library and does not load files. Thus it does not know,
weather DST was in effect at some time...

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						 Jan 'Bulb' Hudec <[e-mail]>