Problem description
As can be seen here, the error ValueError: Could not convert object to NumPy datetime is being thrown by:
df = DataFrame(dict(A=Timestamp('20130101'), B=np.arange(5)))
result = df.groupby('A')['A'].transform(lambda x: x.max() - x.min())
NOTE: I will be submitting a PR shortly.
Output of pd.show_versions()
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little
pandas: 0.19.2
nose: 1.3.7
pip: 8.1.2
setuptools: 18.2
Cython: None
numpy: 1.12.0
dateutil: 2.6.0
pytz: 2016.6.1
Comment From: jreback
this is a dupe of #10972