title: Manipulate list column of dates with dplyr
body: <p>I have a dataframe that looks like this:</p>
  <pre><code>&gt; data
  Simple feature collection with 176 features and 2 fields
  geometry type:  POLYGON
  dimension:      XY
  bbox:           xmin: 4358985 ymin: 2571551 xmax: 4505751 ymax: 2663100
  projected CRS:  Lambert_Azimuthal_Equal_Area
  First 10 features:
      id  slide_dates                       geometry
  1   35        12976 POLYGON ((4481002 2663025, ...
  2   62        10277 POLYGON ((4472778 2661600, ...
  3   82        11400 POLYGON ((4477953 2660650, ...
  4   95         8520 POLYGON ((4476978 2658650, ...
  5   97        14115 POLYGON ((4474878 2658775, ...
  6  166        12017 POLYGON ((4445105 2656175, ...
  7  198        10273 POLYGON ((4472753 2655225, ...
  8  351         7073 POLYGON ((4467303 2651675, ...
  9  377 10149, 16092 POLYGON ((4464129 2652250, ...
  10 426        13685 POLYGON ((4432231 2650525, ...
  </code></pre>
  <p>The column <code>slide_dates</code> is a list with dates stores as integer days after 1970. I would like to replace every element in each list with its &quot;correct&quot; date. I tried several approaches, like this one:</p>
  <pre><code>data %&gt;% 
      mutate(slide_dates = as.Date.numeric(slide_dates, origin=&quot;1970-01-01&quot;))
  </code></pre>
  <p>but I get the following error</p>
  <pre><code>Fehler: Problem with `mutate()` input `slide_dates`.
  x nicht-numerisches Argument für binären Operator
  i Input `slide_dates` is `as.Date.numeric(slide_dates, origin = &quot;1970-01-01&quot;)`
  </code></pre>
  <p>And I'm not really sure how to solve this problem. Any help is appreciated:)</p>
  <p><strong>Some example Data</strong></p>
  <p>This gives the same error</p>
  <pre><code>df = data.frame(a=1:2)
  df$b = list(c(1), c(2, 3))
  df %&gt;% mutate(b = as.Date.numeric(b, origin=Sys.Date()))
  </code></pre>