BUG: nanoseconds argument for DateOffset results in wrong output
```python
>>>ts = pd.Timestamp('2020-1-1')
>>>ts + pd.DateOffset(nanoseconds=1000)
Timestamp('2020-01-02 00:00:00')
```
This is related to `offset = timedelta(1)` in `_determine_offset` from `pd._libs.tslibs.offsets`.