title: Bucketing dates and checking if between date range in pandas
body: <p>How can I check what category a date falls into if it is between a the dates in the date field? I cannot use <code>merge_asof</code> as the work; <code>pandas</code> is only v0.18.</p>
  
  <pre><code>d = {'buckets': ['1D', '1W', '1M'], 'dates': ['03-05-2018', '10-05-2018', '03-06-2018']}
  date_buckets = pd.DataFrame(data=d)
  
  
      buckets dates
  0   1D  03-05-2018
  1   1W  10-05-2018
  2   1M  03-06-2018
  </code></pre>
  
  <p>So, for example, if given the date 07-05-2018, how can I return 1W? I would need to do this for hundreds of rows so would need to be efficient. </p>
  
  <p>thanks,</p>