code: public Calendar floor(long t) {
          Calendar cal = new GregorianCalendar(Locale.US);
          cal.setTimeInMillis(t);
          return floor(cal);
      }
docs: Computes the nearest past timestamp that matched this cron tab.
  <p>
  More precisely, given the time 't', computes another smallest time x such that:
  
  <ul>
  <li>x &lt;= t (inclusive)
  <li>x matches this crontab
  </ul>
  
  <p>
  Note that if t already matches this cron, it's returned as is.