title: Initialize dataframe with sample datetime values
body: <p>I saw a few questions on stackoverflow where sample dataframes where provided but the dates where formated as string. Therefore I wondered if there is a way to initialize the <code>pandas.DataFrame()</code> in a way to already load the data as datetime object in contrast to formating the pd.Series() after initializing the dataframe, as shown below:</p>
  <pre><code>import pandas as pd
  df = pd.DataFrame([['2020-01-01'], ['2020-07-01']], columns=['date'])
  df['date'] = pd.to_datetime(df['date'], format='%Y-%m-%d')
  </code></pre>
  <p>How can I initialize the df without needing the last line?</p>
  <p>I tried:</p>
  <pre><code>pd.DataFrame([['2020-01-01'], ['2020-07-01']], columns = ['date'], dtype=pd.datetime)
  </code></pre>
  <p>which obviously failed.</p>
  <p>Expected output:</p>
  <pre><code>df['date']
  Out[139]: 
  0   2020-01-01
  1   2020-07-01
  Name: date, dtype: datetime64[ns]
  </code></pre>