title: How to create new column with calculation of days between date in 2 other columns in DataFrame in Pandas Python?
body: <p>I have DataFrame in Python Pandas like below:</p>
  <pre><code>col1                | col2
  --------------------|--------------------
  2020-01-10 19:45:49 | 2020-01-11 00:00:00
  2020-01-24 20:14:33 | 2020-01-24 00:00:00
  2020-01-24 11:43:15 | 2020-01-20 00:00:00
  NaN                 | 2020-08-14 00:00:00
  ...                 | ...
  </code></pre>
  <ul>
  <li>col1 is 'object' data type and could have 'NaN' values</li>
  <li>col2 is 'object' data type and does not have 'NaN' values</li>
  </ul>
  <p>I need to create col3 which will present ow many days have passed since the date in col2 (col1 - col2):</p>
  <ul>
  <li>if in col1 is NaN in col3 also should be NaN</li>
  <li>if value in col2 is higher than in col1, value in col3 should be negative</li>
  <li>if value in col2 is lowe than in col1, value in col3 should be positive</li>
  <li>if the value in col2 and col1 is the same, value in col3 should be 0</li>
  </ul>
  <p>So as a result I need something like below:</p>
  <pre><code>col1                | col2                | col3
  --------------------|---------------------|----
  2020-01-10 19:45:49 | 2020-01-11 00:00:00 | -1
  2020-01-24 20:14:33 | 2020-01-24 00:00:00 | 0
  2020-01-24 11:43:15 | 2020-01-20 00:00:00 | 4
  NaN                 | 2020-08-14 00:00:00 | NaN
  ...                 | ...                 | ...
  </code></pre>
  <p>How can I do that in Python Pandas ?</p>