I guess, but I'd prefer to have a single interface for DateOffsets. If having flexibility with offsets means moving some of it out of the Cython code and into methods that's fine, but then you need to do it for all of them. I'd probably prefer you have:

```python
class CythonizedOffset(DateOffset):
```

that has those offset methods with tslib as described and then DateOffset have those as notimplemented, then change all the existing offsets (except, I guess 52-53 week) to inherit from CythonizedOffset instead.