I must be missing something..

The date is always 1 day behind when formating like so:

datetime xDate = o.campaign_start_date__c;
			
String xxDate = xDate.format('yyyy-dd-MM');
system.debug('sdate :' +xxDate);

I set the above field to 8/16/2011

and in debug it shows up as: 2011-15-08

Everytime I change it the debug value is shown as 1 day behind.. Im sure I could to +1, but could someone help me understand why this is happening?

Thanks!
Jacob Davis