title: df.replace not having any effect when trying to replace dates in pandas dataframe
body: <p>I've been through the various comments on here about df.replace but I'm still not able to get it working.</p>
  
  <p>Here is a snippet of my code:</p>
  
  <pre><code># Name columns
  df_yearly.columns = ['symbol', 'date', ' annuual % price change']
  
  # Change date format to D/M/Y
  df_yearly['date'] = pd.to_datetime(df_yearly['date'], format='%d/%m/%Y')
  </code></pre>
  
  <p>The df_yearly dataframe looks like this:</p>
  
  <pre><code>    | symbol |    date    |  annuual % price change 
   ---|--------|------------|------------------------- 
    0 | APX    | 12/31/2017 |                         
    1 | APX    | 12/31/2018 |            -0.502554278 
    2 | AURA   | 12/31/2018 |            -0.974450706 
    3 | BASH   | 12/31/2016 |            -0.998110828 
    4 | BASH   | 12/31/2017 |             8.989361702 
    5 | BASH   | 12/31/2018 |            -0.083599574 
    6 | BCC    | 12/31/2017 |             121718.9303 
    7 | BCC    | 12/31/2018 |            -0.998018734 
  </code></pre>
  
  <p>I want to replace all dates of 12/31/2018 with 06/30/2018.  The next section of my code is:</p>
  
  <pre><code># Replace 31-12-2018 with 30-06-2018 as this is final date in monthly DF
  df_yearly_1 = df_yearly.date.replace('31-12-2018', '30-06-2018')
  print(df_yearly_1)
  </code></pre>
  
  <p>But the output is still coming as:</p>
  
  <pre><code>   | 0 |  2017-12-31 
     | 1 |  2018-12-31 
     | 2 |  2018-12-31 
     | 3 |  2016-12-31 
     | 4 |  2017-12-31 
     | 5 |  2018-12-31 
  </code></pre>
  
  <p>Is anyone able to help me with this?  I thought this might be related to me having the date format incorrect in my df.replace statement but I've tried to search and replace 12-31-2018 and it's still not doing anything.</p>
  
  <p>Thanks in advance!!</p>