title: pandas conditional str concat with a Datetime column
body: <p>I would like to join only the day-ahead with the year of my Date column to make it look like day-ahead_2019 or day-ahead_2018 depending on the year in Date column</p>
  
  <p>I have two columns: </p>
  
  <pre><code>DELIVERY_PERIOD   
  day-ahead
  day-ahead
  day-ahead
  day-ahead
  week1_2019
  week2_2018
  </code></pre>
  
  <p>and, </p>
  
  <pre><code>Date
  13/05/2019
  14/05/2019
  11/05/2019
  10/05/2019
  </code></pre>
  
  <p>I tried to use str.cat but it doesnt work for datetime64 for my Date column</p>
  
  <pre><code>       df.DELIVERY_PERIOD == 'day-ahead' = df.str.cat(df.Date.dt.year)
  </code></pre>
  
  <p>error came up as it is not str type data
  i thought about the other way around
  which is create new columns to get year from the Date.dt.year, convert it to str and only join with day-ahead
  but i think this would be a long way around
  i welcome any suggestions</p>