In [7]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg('size')
Out[7]: 
1    2
2    2
dtype: int64

In [8]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg(len)
Out[8]: 
1   1970-01-01 00:00:00.000000002
2   1970-01-01 00:00:00.000000002
dtype: datetime64[ns]

In [9]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg(numpy.size)
Out[9]: 
1   1970-01-01 00:00:00.000000002
2   1970-01-01 00:00:00.000000002
dtype: datetime64[ns]

Output of pd.show_versions()

commit: 3853fe6d2b884f186b93933ef53ff5e475c2d80c python: 2.7.11.final.0 python-bits: 64 OS: Linux OS-release: 4.4.0-59-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: None.None pandas: 0.19.0+395.g3853fe6 nose: 1.3.7 pip: 8.1.2 setuptools: 22.0.0 Cython: 0.24 numpy: 1.11.0 scipy: 0.17.1 statsmodels: None xarray: None IPython: 4.2.0 sphinx: None patsy: None dateutil: 2.5.3 pytz: 2016.4 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: None openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: None httplib2: None apiclient: None sqlalchemy: None pymysql: None psycopg2: None jinja2: None s3fs: None pandas_datareader: None

Comment From: jreback

this is a dupe: https://github.com/pandas-dev/pandas/issues/10972

welcome to have a pr to fix.