cdm/src/main/java/ucar/nc2/time/CalendarDateUnit.java
static public CalendarDateUnit of(Calendar calt, CalendarPeriod.Field periodField, CalendarDate baseDate) {
    if (calt == null) calt = Calendar.getDefault();
    return new CalendarDateUnit(calt, periodField, baseDate);
  }
static public CalendarDateUnit of(Calendar calt, CalendarPeriod.Field periodField, CalendarDate baseDate) {
    if (calt == null) calt = Calendar.getDefault();
    return new CalendarDateUnit(calt, periodField, baseDate);
  }
Create a CalendarDateUnit from a calendar, a CalendarPeriod.Field, and a base date
@param calt use this Calendar, or null for default calendar
@param periodField a CalendarPeriod.Field like Hour or second
@param baseDate "since baseDate"
@return CalendarDateUnit