Pandas 0.23.4: when does "ValueError: Values falls before first bin" happens during grouping?
I didn't find any reference of a Pandas error that happens during grouping: `"ValueError: Values falls before first bin"`.
Can it be due to a internal bug?

According to my opinion it should never happen that a group cannot be done. None should be returned instead.

The trace is:

      File "new_calc.py", line 584, in calculate_groups
        grouped = df.groupby(pd.Grouper(freq="D"))
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/generic.py", line 6665, in groupby
        observed=observed, **kwargs)
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/groupby/groupby.py", line 2152, in groupby
        return klass(obj, by, **kwds)
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/groupby/groupby.py", line 599, in __init__
        mutated=self.mutated)
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/groupby/groupby.py", line 3189, in _get_grouper
        binner, grouper, obj = key._get_grouper(obj, validate=False)
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/resample.py", line 1281, in _get_grouper
        r._set_binner()
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/resample.py", line 148, in _set_binner
        self.binner, self.grouper = self._get_binner()
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/resample.py", line 156, in _get_binner
        binner, bins, binlabels = self._get_binner_for_time()
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/resample.py", line 888, in _get_binner_for_time
        return self.groupby._get_time_bins(self.ax)
      File "/etc/bluelikon/utilities/env/local/lib/python2.7/site-packages/pandas/core/resample.py", line 1333, in _get_time_bins
        ax_values, bin_edges, self.closed, hasnans=ax.hasnans)
      File "pandas/_libs/lib.pyx", line 554, in pandas._libs.lib.generate_bins_dt64
    ValueError: Values falls before first bin
