grib/src/main/java/ucar/nc2/grib/coord/CoordinateTime2DUnionizer.java
void setRuntimeCoords(CoordinateRuntime runtimes) {
    for (int idx=0; idx<runtimes.getSize(); idx++) {
      CalendarDate cd = runtimes.getRuntimeDate(idx);
      long runtime = runtimes.getRuntime(idx);
      CoordinateTimeAbstract time = timeMap.get(runtime);
      if (time == null) {
        time = isTimeInterval ? new CoordinateTimeIntv(this.code, this.timeUnit, cd, new ArrayList<>(0), null) :
                new CoordinateTime(this.code, this.timeUnit, cd, new ArrayList<>(0), null);
        timeMap.put(runtime, time);
      }
    }
  }
void setRuntimeCoords(CoordinateRuntime runtimes) {
    for (int idx=0; idx<runtimes.getSize(); idx++) {
      CalendarDate cd = runtimes.getRuntimeDate(idx);
      long runtime = runtimes.getRuntime(idx);
      CoordinateTimeAbstract time = timeMap.get(runtime);
      if (time == null) {
        time = isTimeInterval ? new CoordinateTimeIntv(this.code, this.timeUnit, cd, new ArrayList<>(0), null) :
                new CoordinateTime(this.code, this.timeUnit, cd, new ArrayList<>(0), null);
        timeMap.put(runtime, time);
      }
    }
  }
set the list of runtime coordinates; add any that are not already present, and make an empty CoordinateTimeAbstract for it