Try the following:

• Locate the config folder, which is a sub-folder off of the $INSTALL_DIR
• Create/modify server.properties file - ensure the name is server.properties and not service.properties or server.properties.txt
• Add the following line:
  
  user.timezone=America/Los_Angeles
  
  
• Save the file and restart Xeam
• To confirm Xeams accepted this change, go to the About screen under Tools and search for user.timezone. It should display the modified value