code: public T minus(
          long amount,
          U unit
      ) {
  
          return this.plus(MathUtils.safeNegate(amount), unit);
  
      }
docs: /*[deutsch]
  <p>Subtrahiert den angegebenen Betrag der entsprechenden Zeiteinheit
  von dieser Bezugszeit und liefert das Ergebnis zur&uuml;ck. </p>
  
  @param   amount      amount to be subtracted (maybe negative)
  @param   unit        time unit
  @return  result of subtraction as changed copy, this instance
  remains unaffected
  @throws  RuleNotFoundException if given 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(long, Object) plus(long, U)