html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/364#issuecomment-341598412,https://api.github.com/repos/pydata/xarray/issues/364,341598412,MDEyOklzc3VlQ29tbWVudDM0MTU5ODQxMg==,14136435,2017-11-03T00:40:14Z,2017-11-03T00:40:39Z,CONTRIBUTOR,"Hi, being able to pass a `pd.TimeGrouper` to `.groupby()` would be really handy. Here is my use-case and work around at the moment (`.resample()` doesn't serve my needs because I need to iterate over the groups):

```python
import pandas as pd
import xarray as xr

dates = pd.DatetimeIndex(['2017-01-01 15:00', '2017-01-02 14:00', '2017-01-02 23:00'])
da = xr.DataArray([1, 2, 3], dims=['time'], coords={'time': dates})
time_grouper = pd.TimeGrouper(freq='24h', base=15)

# digging around the source code for xr.DataArray.resample i found this
grouped = xr.core.groupby.DataArrayGroupBy(da, 'time', grouper=time_grouper)

for _, sub_da in grouped:
    print(sub_da)
```

which prints: 
```
<xarray.DataArray (time: 2)>
array([1, 2])
Coordinates:
  * time     (time) datetime64[ns] 2017-01-01T15:00:00 2017-01-02T14:00:00
<xarray.DataArray (time: 1)>
array([3])
Coordinates:
  * time     (time) datetime64[ns] 2017-01-02T23:00:00
```

Would it be possible to add a `grouper` kwarg to `.groupby()`, e.g.
```python
da.groupby('time', grouper=time_grouper)
```
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