Hi,
registerd users can set timeoffset, but its rounded when user clicks on autodetect and not showing exact timeoffset. If u want to show exact timeoffset. use the following code.
In default theme go to profile.template.php
Replace the following code
var diff = Math.round((localTime.getTime() - serverTime.getTime())/3600000);
with.
var diff = (localTime.getTime() - serverTime.getTime())/3600000;
diff=diff.toString();
diff=diff.substring(0,6);
diff=parseFloat(diff);
Now this will show exact timeoffset..