title: Merging columns to create a date with a header
body: <p>I'm using a large climate file where there's a YYYY, MM, DD columns. I want to merge these to create a date column, while keep all the original data in the dataset. </p>
  
  <p>I have managed to do this so far which gets me almost what I want but I can't seem to get a header in the date column. </p>
  
  <pre><code>climate = pd.read_csv(r'R:\Climate\SILO\PatchedPoint\Current_csv\86090.csv')
  
  climate.apply(pd.to_numeric, errors = 'ignore')
  climate_nozero = climate.drop([0])
  
  climate2 = climate_nozero.rename(columns = {'YYYY':'Year','MM':'Month','DD':'Day'})
  
  index = climate2.apply(lambda x: pd.datetime.strptime("{0} {1} {2}".format(x['Year'],x['Month'], x['Day']), "%Y %m %d"),axis=1) 
  
  climate3 = pd.concat([index, climate2], axis=1)
  </code></pre>
  
  <p>I tried </p>
  
  <pre><code>climate4 = climate3.rename(columns = {'0':'Date'})
  </code></pre>
  
  <p>to change the header but it does nothing </p>
  
  <p><a href="https://i.stack.imgur.com/R4uid.png" rel="nofollow noreferrer">I've added the output table that I get</a></p>