add a DATE with a NUMBER and to obtain a DATE in R : <p>I have a data frame "Invoice" which look like this:</p>

<pre><code>Invoice_ID    Invoice_DATE   Nr_of_days_until_deadline
  101           1/20/2017         7
  102           1/25/2017         4
  103           1/29/2017         5
  104           2/01/2017         4
  105           2/05/2017         3
</code></pre>

<p>I have to populate the  data frame Deadline_Invoces by writing  R modules   for determining the deadline DATE   for each    invoice,    where deadline_DATE =   Invoice_DATE    +   Nr_of_days_until_deadline.</p>

<p>Thus I have to obtain data frame Deadline_Invoces:  </p>

<pre><code>    Invoice_ID  Invoice_DATE    Deadline_DATE
</code></pre>

<p>How should I sum DATE with a NUMBER and to obtain a DATE?</p>

<p>Thank you in advance!!! </p>
