title: numpy and pandas timedelta error
body: <p>In Python I have an array of dates generated (or read from a CSV-file) using pandas, and I want to add one year to each date. I can get it working using pandas but not using numpy. What am I doing wrong? Or is it a bug in either pandas or numpy?</p>
  
  <p>Thanks!</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  from pandas.tseries.offsets import DateOffset
  
  # Generate range of dates using pandas.
  dates = pd.date_range('1980-01-01', '2015-01-01')
  
  # Add one year using pandas.
  dates2 = dates + DateOffset(years=1)
  
  # Convert result to numpy. THIS WORKS!
  dates2_np = dates2.values
  
  # Convert original dates to numpy array.
  dates_np = dates.values
  
  # Add one year using numpy. THIS FAILS!
  dates3 = dates_np + np.timedelta64(1, 'Y')
  
  # TypeError: Cannot get a common metadata divisor for NumPy datetime metadata [ns] and [Y] because they have incompatible nonlinear base time units
  </code></pre>