PHP timezone is unstable...

Hello,

what could cause the timezone in PHP to, randomly, jump back 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