title: pandas: create a new date time column by combining existing columns
body: <p>I am trying to create a new datetime column from existing columns (one datetime column and another integer column) in a pandas data frame. Here is my code:</p>
  
  <pre><code>import datetime
  df['start_date'] = pd.to_datetime(df['start_date'])
  df['end_date'] = df['start_date'] + pd.Timedelta(df.total_waiting_days, unit='D')
  </code></pre>
  
  <p>But I got the following errors:</p>
  
  <pre><code>ValueError: Value must be Timedelta, string, integer, float, timedelta or convertible
  </code></pre>
  
  <p>What did I do wrong here and how do I fix this? Thanks!</p>