cdm/src/main/java/ucar/nc2/time/CalendarDateUnit.java
static public CalendarDateUnit withCalendar(Calendar calt, String udunitString) {
    if (calt == null) calt = Calendar.getDefault();
    return new CalendarDateUnit(calt, udunitString);
  }
static public CalendarDateUnit withCalendar(Calendar calt, String udunitString) {
    if (calt == null) calt = Calendar.getDefault();
    return new CalendarDateUnit(calt, udunitString);
  }
Create a CalendarDateUnit from a calendar and a udunit string = "unit since calendarDate"
@param calt use this Calendar, or null for default calendar
@param udunitString "unit since calendarDate"
@return CalendarDateUnit
@throws IllegalArgumentException if udunitString is not paresable