PHP timezone is unstable...

Hello, what could cause the timezone in PHP to, randomly, jump back and forth 6 hours now and then? from phpinfo(): date

date/time support     enabled
"Olson" Timezone Database Version     0.system
Timezone Database     internal
Default timezone     America/Chicago

Directive    Local Value    Master Value
date.default_latitude    31.7667    31.7667
date.default_longitude    35.2333    35.2333
date.sunrise_zenith    90.583333    90.583333
date.sunset_zenith    90.583333    90.583333
date.timezone    Europe/Sweden    Europe/Sweden

"Default timezone" jumps between "America/Chicago" and "Europe/Berlin" while "date.timezone" stays "Europe/Sweden" all the time... Dirk