title: Convert/use timedelta's time to datetime
body: <p>I have a dataframe with two columns: 1 timedelta 'Time', and 1 datetime 'DateTime'.</p>
  
  <p>My timedelta column simply contains/displays a normal regular time, it never has more than 24 hours. It's not being used as a 'timedetla', just 'time'.
  It's just the way it comes when pandas gets the data from my database.</p>
  
  <p>I want a new column 'NewDateTime', with the date from the datetime, and time from the deltatime.</p>
  
  <p>So I have this:</p>
  
  <pre><code>        Time       DateTime     
  1       09:01:00   2018-01-01 10:10:10
  2       21:43:00   2018-01-01 11:11:11
  3       03:20:00   2018-01-01 12:12:12
  </code></pre>
  
  <p>And I want this:</p>
  
  <pre><code>        Time       DateTime                NewDateTime
  1       09:01:00   2018-01-01 10:10:10     2018-01-01 09:01:00
  2       21:43:00   2018-01-01 11:11:11     2018-01-01 21:43:00
  3       03:20:00   2018-01-01 12:12:12     2018-01-01 03:20:00
  </code></pre>
  
  <p>At first I tried to set the DateTime column's hours, minutes and seconds to 0.
  Then I planned to add the timedelta to the datetime.</p>
  
  <p>But when I tried to do:</p>
  
  <p><code>df['NewDateTime'] = df['DateTime'].dt.replace(hour=0, minute=0, second=0)</code></p>
  
  <p>I get <code>AttributeError: 'DatetimeProperties' object has no attribute 'replace'</code></p>