title: AM/PM to TimeSpan
body: <p>I want to achieve the converse of <a href="https://stackoverflow.com/questions/6724040/how-to-convert-timespan-to-pm-or-am-time">this</a>, that is, I want to convert a <code>string</code> with format <code>hh:mm tt</code> to a <code>TimeSpan</code> with zeroed seconds.</p>
  
  <p>For example, <code>09:45 pm</code> is converted to <code>21:45:00</code>.</p>