code: public T minus(TimeSpan<? extends U> timeSpan) {
  
          try {
              return timeSpan.subtractFrom(this.getContext());
          } catch (IllegalArgumentException iae) {
              ArithmeticException ex = new ArithmeticException("Result beyond boundaries of time axis.");
              ex.initCause(iae);
              throw ex;
          }
  
      }
docs: /*[deutsch]
  <p>Subtrahiert die angegebene Zeitspanne von der Bezugszeit und
  liefert das Subtraktionsergebnis zur&uuml;ck. </p>
  
  <p>Delegiert an {@link TimeSpan#subtractFrom(TimePoint)}. </p>
  
  @param   timeSpan    time span to be subtracted from this instance
  @return  result of subtraction as changed copy, this
  instance remains unaffected
  @throws  RuleNotFoundException if any time unit is not registered
  and does also not implement {@link BasicUnit} to yield
  a suitable unit rule for the underlying time axis
  @throws  ArithmeticException in case of numerical overflow
  @see     #plus(TimeSpan)