title: How to split year from date and make a new column; how to deal with leap years
body: <p>I am very new to coding (this is the first code I am writing). </p>
  
  <p>I have multiple csv files, all with the same headers. The files correspond to hourly ozone concentration for every day of the year, and each file is a separate year [range from 2009-2020]. I have a column 'date' that contains the year-month-day, and I have a column for hour of the day (0-23). I want to separate the year from the month-day, combine the hour with the month-day and make this the index, and then merge the other csv files into one dataframe.</p>
  
  <p>In addition, I need to average data values from each day at each hour for all 10 years, however, three of my files include leap days (an extra 24 values). I would appreciate any advice on how to account for the leap years. I assume that I would need to add the leap day to the files without it, then provide null values, then drop the null values (but that seems circular).</p>
  
  <p>Also, if you have any tips on how to simplify my process, feel free to share!</p>
  
  <p>Thanks in advance for your help.</p>
  
  <p><strong>Update</strong>: I tried the advice from Rookie below, but after importing csv data, I get an error message:</p>
  
  <pre><code>import pandas as pd
  import os
  </code></pre>
  
  <pre><code>path = "C:/Users/heath/Documents/ARB project Spring2020/ozone/SJV/SKNP"
  </code></pre>
  
  <pre><code>df = pd.DataFrame()
  for file in os.listdir(path):
      df_temp = pd.read_csv(os.path.join(path, file))
      df = pd.concat((df, df_temp), axis = 0)
  </code></pre>
  
  <p>First, I get an error message that says <code>OSError: Initializing from file failed</code>.
  I tried to fix the issue by adding <code>engine = 'python'</code> based on advice from <a href="https://stackoverflow.com/questions/50552404/oserror-initializing-from-file-failed-on-csv-in-pandas">OSError: Initializing from file failed on csv in Pandas</a>, but now I'm getting <code>PermissionError: [Errno 13] Permission denied: 'C:/Users/heath/Documents/ARB project Spring2020/ozone/SJV/SKNP\\.ipynb_checkpoints'</code></p>
  
  <p>Please help, I'm not sure what else to do. I edited the permission so that everyone has the read &amp; write access. However, I still had the "permission denied" error when I imported the csv on Windows.</p>