Hi Timo,
as you linked to the Dataset interface (and not the Entity): You get the last change date from a Dataset via the .getLastChanged() method which returns a long value (a timestamp). It's defined in the StoreElement interface from which a Dataset inherits.
If you happen to just have an Entity, you can get the corresponding Dataset object either via the corresponding Content2 (=Datasource) or TableTemplate using their .getDataset(Entity) methods before.
Best,
Michael