Hi Rafik,
Tunis shouldn't be using DST by default. If you check the zoneinfo for this timezone, DST (isdst) is set to 0 for the future:
zdump -v /usr/share/zoneinfo/Africa/Tunis | tail -3
/usr/share/zoneinfo/Africa/Tunis Sun Oct 26 01:00:00 2008 UTC = Sun Oct 26 02:00:00 2008 CET isdst=0 gmtoff=3600
/usr/share/zoneinfo/Africa/Tunis Mon Jan 18 03:14:07 2038 UTC = Mon Jan 18 04:14:07
2038 CET isdst=0 gmtoff=3600
/usr/share/zoneinfo/Africa/Tunis Tue Jan 19 03:14:07 2038 UTC = Tue Jan 19 04:14:07
2038 CET isdst=0 gmtoff=3600
If you want to set the timezone to UTC (which doesn't use DST either), you can check SOL8551:
sol8551: F5 time zone update policy
http://support.f5.com/kb/en-us/solutions/public/8000/500/sol8551.html
Aaron