+ignoring closed days. Always returns a positive number irrespective of the
+relative order of the parameters
+
+=head2 next_open_day
+
+$datetime = $calendar->next_open_day($duedate_dt)
+
+Passed a Datetime returns another Datetime representing the next open day. It is
+intended for use to calculate the due date when useDaysMode syspref is set to either
+'Datedue' or 'Calendar'.
+
+=head2 prev_open_day
+
+$datetime = $calendar->prev_open_day($duedate_dt)
+
+Passed a Datetime returns another Datetime representing the previous open day. It is
+intended for use to calculate the due date when useDaysMode syspref is set to either
+'Datedue' or 'Calendar'.
+
+=head2 set_daysmode
+
+For testing only allows the calling script to change days mode
+
+=head2 clear_weekly_closed_days
+
+In test mode changes the testing set of closed days to a new set with
+no closed days. TODO passing an array of closed days to this would
+allow testing of more configurations
+
+=head2 add_holiday
+
+Passed a datetime object this will add it to the calendar's list of
+closed days. This is for testing so that we can alter the Calenfar object's
+list of specified dates