```
def nocheck(x):
    return np.asarray(x, dtype='datetime64[ns]')

def check(x):
    if not x.dtype == '<M8[ns]':
        return np.asarray(x, dtype='datetime64[ns]')
    else:
        return x

In [18]: %timeit check(d)
1000000 loops, best of 3: 373 ns per loop  
In [19]: %timeit nocheck(d)
1000000 loops, best of 3: 1.37 us per loop
```