i don't think `timedelta64` parses arbitrary things....only quirk is that strings are allowed

```
In [48]: timedelta64('2013','D')
Out[48]: numpy.timedelta64(2013,'D')

In [49]: timedelta64(2013,'D')
Out[49]: numpy.timedelta64(2013,'D')

In [50]: timedelta64('2013-01-01','D')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-50-1fa166bb54b5> in <module>()
----> 1 timedelta64('2013-01-01','D')

ValueError: Could not convert object to NumPy timedelta

In [51]: datetime64('2013-01-01')
Out[51]: numpy.datetime64('2013-01-01')
```