title: ValueError: You are trying to merge on datetime64[ns] and object columns. If you wish to proceed you should use pd.concat
body: <p>I want to make a pivot table and fill to it in Date.I wrote codes.</p>
  
  <pre><code>user_dct ={100:"Tom",101:"Jon",102:"Daisy"}
  for key,value in user_dct.items():
          　file= './'+value+'.csv'
      　　 df = pd.read_csv(file)
  </code></pre>
  
  <p>Each df is</p>
  
  <pre><code>    Date        ID        Name    Score        Rank
  0    2011-01-12  100     Tom        40            C
  1    2011-01-14  100     Tom        60            B
  2    2011-01-19  100     Tom        80            A
  ・
  ・
  ・
  
  
     Date        ID        Name    Score        Rank
  0    2011-01-12  101     Jon        30            C
  1    2011-01-14  101     Jon        50            C
  2    2011-01-19  101     Jon        60            B
  ・
  ・
  ・
  
  user_dct ={100:"Tom",101:"Jon",102:"Daisy"}
  dfs = []
  for key,value in user_dct.items():
      file= './'+value+'.csv'
      dfs.append(pd.read_csv(file, parse_dates=['Date']))
  
  df = pd.concat(dfs, ignore_index=True)
  df =df.sort_values(['Date','ID']).set_index(['Date','ID'])
  
  date_df = pd.DataFrame({'Date':pd.date_range('2011-01-01','2011-12-31',freq='1D').strftime('%Y-%m-%d')})
  df = pd.merge(df, date_df, on='Date', how='outer').fillna(0)
  </code></pre>
  
  <p>My ideal output is</p>
  
  <pre><code>                        Name    Score        Rank
      Date    　　ID  
  2011-01-01  100         Tom       0              0
                      101         Jon        0              0
                      102     Daisy       0              0  
                      ・
                                           ・
                                           ・
  2011-01-12  100         Tom       40            C
                      101         Jon        30            C
                      102     Daisy       90            S
  2011-01-14  100     Tom         60            B
                      101     Jon           50            C
                       102     Daisy       90            S
  2011-01-19  100     Tom         80            A
                       101     Jon          60            B
                       102     Daisy      80            A
  ・
  ・
  ・
  </code></pre>
  
  <p>What is wrong in my codes?How should I fix this?Why does int type error happens?I changed sort_values&amp;set_index ,but error is not disappeared.</p>