nshlib/nsh_timcmds.c: fix incorrect time set during summer time

[michallenc]

Summary

Value zero in tm_isdst means daylight saving time (or summer time) is not in effect. This is obviously not true for half of the year in most timezones and keeping it at zero leads to incorrect time being set. This commit sets tm_isdst to -1, which means the information is not available and it should be handled according to timezone rules.

Impact

Fixes time set with date command.

Testing

On custom device with Europe/Prague (UTC+2 now in summer time, otherwise UTC+1) timezone set.

Before the fix -> both local and UTC time not correctly set:

vtk25> date -s "Sep 15 11:30:00 2023"
vtk25> date
Fri, Sep 15 12:30:01 2023
vtk25> date -u
Fri, Sep 15 10:30:02 2023

After the fix (tested both with and without daylight saving):

vtk25> date -s "Sep 15 11:30:00 2023"
vtk25> date
Fri, Sep 15 11:30:01 2023
vtk25> date -u
Fri, Sep 15 09:30:05 2023

vtk25> date -s "Jan 15 11:30:00 2023"
vtk25> date
Sun, Jan 15 11:30:01 2023
vtk25> date -u
Sun, Jan 15 10:30:03 2023
vtk25> 

Tests if no timezone is uploaded

vtk25> date -s "Sep 15 11:30:00 2023"
vtk25> date
Fri, Sep 15 11:30:02 2023
vtk25> date -u
Fri, Sep 15 11:30:04 2023
vtk25> 

vtk25> date -s "Jan 15 11:30:00 2023"
vtk25> date
Sun, Jan 15 11:30:01 2023
vtk25> date -u
Sun, Jan 15 11:30:02 2023
vtk25> 

[acassis]

@michallenc nice finding!