Step 1: Make sure you're in sync with portage.

emerge --sync

Step 2: Make sure you're up to date with timezone information.

emerge -av timezone-data

Step 3: Make sure your timezone info is set and up to date:

cp /usr/share/zoneinfo/America/Montreal /etc/localtime

NB Of course, use the zoneinfo file that fits where you are!

Step 4: Restart proper services

/etc/init.d/mysql restart
/etc/init.d/apache2 restart
/etc/init.d/tomcat5 restart

and any other you can thing of... or reboot, if you have that kind of luxury.