code: private void setCombinedValue(UTCDateBox date, UTCTimeBox time, long value, boolean setTimeValue) {
          date.setValue(datePartMillis(value), false);        
          if (setTimeValue) {
              time.setValue(timePartMillis(value), false);
          }
      }
docs: This allows us to treat a datetime as a single value, making it
  easy for comparison and adjustment. We don't actually expose
  this because the timezone issues make it too confusing to
  clients.
  
  @param setTimeValue
  Sometimes we don't want to set the time value
  explicitly. Generally this is the case when we
  haven't specified a time.