I had this problem and fixed it by checking the phpinfo ( Reports -> System Info -> phpinfo ). Look for the Default Timezone. I copied this to the Modx system setting, I'm actually 1 hour ahead of that time so I added a +1 to the server_offset_time setting. I did empty the core/cache folder. Flushed permissions and it worked.
I did check the variables in MySQL and time zone there is set to SYSTEM. That's what it should be I think.
If it's something else maybe try a php.ini file in the root of Modx with:
date.timezone = "US/Central"
or whatever. Make sure that matches the setting in Modx.
Anyway this worked for me. Was very frustrating for a while though.