You can set the time and date on dm800 by following command :
date -s YYYY.MM.DD-hh:mm:ss
YYYY is Year
MM is Month
DD is Day
hh is Hour (0-24)
mm is Minute
ss is Second
for changing the timezone , first get a file list of cities by following command :
ls /usr/share/zoneinfo/
choose your city or nearest city to you , then use the following command to change the timezone ( replace YOUR_CITY with your chosen city ) :
ln -sf /usr/share/zoneinfo/YOUR_CITY /etc/localtime