title: How to create a datetime object from separate date fields?
body: <p>I have a dataset like this:</p>
  
  <pre><code>    Year MM DD HH
  158 2010  7  1  5
  159 2010  7  1  5
  160 2010  7  1  6
  161 2010  7  1  6
  
  structure(list(Year = c(2010L, 2010L, 2010L, 2010L), MM = c(7L, 
  7L, 7L, 7L), DD = c(1L, 1L, 1L, 1L), HH = c(5L, 5L, 6L, 6L)), .Names = c("Year", 
  "MM", "DD", "HH"), row.names = 158:161, class = "data.frame")
  </code></pre>
  
  <p>How can I create a one datetime object from this data set (new column for this data)?</p>