title: Subtract n hours from a DateTime in Ruby
body: <p>I have a Ruby DateTime which gets filled from a form. Additionally I have n hours from the form as well. I'd like to subtract those n hours from the previous DateTime. (To get a time range).</p>
  
  <p>DateTime has two methods "-" and "&lt;&lt;" to subtract day and month, but not hour. (<a href="https://ruby-doc.org/stdlib-2.5.1/libdoc/date/rdoc/DateTime.html" rel="noreferrer">API</a>). Any suggestions how I can do that?</p>