I believe you should be able to, yes, so long as the version of cron that SliTaz has will permit it. Have you tried to put environmental variables into your crontab? Here's an example from the Internet:

TZ=Africa/Abidjan
HOME=/tmp/cg13442
23 * 1-9,11-26,28-29,31 2-10,12 * exec /var/tmp/cron/crontest.sh.1.1 23 \* 1-9,1
1-26,28-29,31 2-10,12 \* Africa/Abidjan
HOME=/var/tmp/cg13442
3 0-7,9-10,12-22 1-6,8-9,11-21,24-26,28 1-7,9-10,12 * exec /var/tmp/cron/crontes
t.sh.1.1 3 0-7,9-10,12-22 1-6,8-9,11-21,24-26,28 1-7,9-10,12 \* Africa/Abidjan
TZ=Africa/Accra
37 0-2,4-5,7-17,20-22 * * * exec /var/tmp/cron/crontest.sh.1.1 37 0-2,4-5,7-17,2
0-22 \* \* \* Africa/Accra

You can see that you can even set more than one time zone. The crontab is read from top to bottom, so a time zone variable is valid until cron reads the next one.

I don't have an installation of SliTaz (only using LiveCD version so far), so I have not tried this on SliTaz myself, but maybe it's a place to start investigating...?

thx marquitico for the info - it works also in SliTaz, but not after a reboot - it still stays on UTC.

I have to restart cron - then it works -strange

Hmmm. We really need an expert to join this conversation, but it sounds like something wrong with the boot sequence. One needs to be certain that when the computer boots up, anything that is time-sensitive must wait until after the system itself sets the timezone. So if it works after a boot up only when you do it yourself, maybe the crond is being started too early in the boot process?