docs: private static Timespan truncateDurationToUnit(long durationInMillis, TimeUnit timeUnit) {
          Timespan res;
  
          if (durationInMillis >= timeUnit.getMillisecondsCount()) {
              res = new Timespan(durationInMillis / timeUnit.getMillisecondsCount(),
                  timeUnit);
          } else {
              res = ZERO_TIMESPANS.get(timeUnit);
          }
  
          return res;
      }