title: Using `mutate_at()` with `as.Date()`
body: <p>I'm trying to use <code>mutate_at()</code> from <code>dplyr</code> to coerce date-like columns into columns of type <code>Date</code> using <code>as.Date()</code>, but I'm getting an error. Here's the code:</p>
  
  <pre><code>library(dplyr)
  
  df = data.frame(date_1 = "7/5/2014", date_2 = "7/22/2011")
  df %&gt;%
      mutate_at(.vars = c("date_1", "date_2"), .funs = as.Date("%m/%d/%Y"))
  </code></pre>
  
  <p>This gives me an error: <code>Error in charToDate(x): character string is not in a standard unambiguous format</code></p>
  
  <p>Not sure what's going on here, so I'd appreciate your help. I prefer <code>dplyr</code> solutions, but if there's a better way to do it, I'm open to that as well.</p>