code: public static int getWeeksSinceEpochFromJulianDay(int julianDay, int firstDayOfWeek) {
          int diff = Time.THURSDAY - firstDayOfWeek;
          if (diff < 0) {
              diff += 7;
          }
          int refDay = Time.EPOCH_JULIAN_DAY - diff;
          return (julianDay - refDay) / 7;
      }
docs: Returns the week since {@link Time#EPOCH_JULIAN_DAY} (Jan 1, 1970)
  adjusted for first day of week.
  <p/>
  This takes a julian day and the week start day and calculates which
  week since {@link Time#EPOCH_JULIAN_DAY} that day occurs in, starting
  at 0. *Do not* use this to compute the ISO week number for the year.
  
  @param julianDay      The julian day to calculate the week number for
  @param firstDayOfWeek Which week day is the first day of the week,
  see {@link Time#SUNDAY}
  @return Weeks since the epoch