This impacts two points:
First : https://github.com/Work4Labs/gooddata-python/blob/mvdb/bring-uptodate/gooddataclient/columns.py#L254
Second : https://github.com/Work4Labs/gooddata-python/blob/mvdb/bring-uptodate/gooddataclient/dataset.py#L118

This makes it possible to consider Date columns as if they were normal columns (but there really are more complex than that. When specifying a date column, you have to create other objects. It's GD style. Yuck.)