title: mutate across (decompose) all dates in a dataframe
body: <p>have a lot of date columns in my dataframe and needed to transform all the dates to three new columns per date (day of month, month and year)</p>
  <p>I try to avoid to specifically write every column down, so i am using <code>across</code> but still wrapping my mind around this kind of verbs</p>
  <pre><code>library(tidyverse)
  
  df &lt;- tibble(dates1 = c(&quot;2020-08-03&quot;, &quot;2020-08-03&quot;),
             dates2 = c(&quot;2020-08-05&quot;, &quot;2020-08-05&quot;))
  
  
  df %&gt;% mutate(across(contains(&quot;dates&quot;), lubridate::day(.), lubridate::month(.), lubridate::year(.) ))
  
  Error: Problem with `mutate()` input `..1`.
  i `..1 = across(...)`.
  x do not know how to convert 'x' to class “POSIXlt”
  </code></pre>
  <p>and guessing how the new columns should be named....I am a bit lost</p>