If passing a DateTime object into the expression it will use the local timezone of the computer running the code.
Please See code below
QueryExpression oQuery = new QueryExpression(fetchEntity);
oQuery.ColumnSet = new ColumnSet(true);
oQuery.Criteria = new FilterExpression(LogicalOperator.And);
oQuery.Criteria.AddCondition(attName,ConditionOperator.OnOrAfter,new DateTime(2015,02,15));
oQuery.PageInfo = new PagingInfo();
oQuery.PageInfo.PageNumber = 1;
bool hasMore = true;
List<Entity> fullList = new List<Entity>();
while(hasMore)
{
EntityCollection oCollection = ServiceProxy.RetrieveMultiple(oQuery);
fullList.AddRange(oCollection.Entities.ToArray());
hasMore = oCollection.MoreRecords;
oQuery.PageInfo.PageNumber += 1;
}
return fullList;