Subclasses provide implementations to expand (or limit) a list of dates based on rule requirements as
specified in RFC5545.
3.3.10. Recurrence Rule
...
BYxxx rule parts modify the recurrence in some manner. BYxxx rule
parts for a period of time that is the same or greater than the
frequency generally reduce or limit the number of occurrences of
the recurrence generated. For example, "FREQ=DAILY;BYMONTH=1"
reduces the number of recurrence instances from all days (if
BYMONTH rule part is not present) to all days in January. BYxxx
rule parts for a period of time less than the frequency generally
increase or expand the number of occurrences of the recurrence.
For example, "FREQ=YEARLY;BYMONTH=1,2" increases the number of
days within the yearly recurrence set from 1 (if BYMONTH rule part
is not present) to 2.
If multiple BYxxx rule parts are specified, then after evaluating
the specified FREQ and INTERVAL rule parts, the BYxxx rule parts
are applied to the current set of evaluated occurrences in the
following order: BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY, BYDAY,
BYHOUR, BYMINUTE, BYSECOND and BYSETPOS; then COUNT and UNTIL are
evaluated.
The table below summarizes the dependency of BYxxx rule part
expand or limit behavior on the FREQ rule part value.
The term "N/A" means that the corresponding BYxxx rule part MUST
NOT be used with the corresponding FREQ value.
BYDAY has some special behavior depending on the FREQ value and
this is described in separate notes below the table.
+----------+--------+--------+-------+-------+------+-------+------+
| |SECONDLY|MINUTELY|HOURLY |DAILY |WEEKLY|MONTHLY|YEARLY|
+----------+--------+--------+-------+-------+------+-------+------+
|BYMONTH |Limit |Limit |Limit |Limit |Limit |Limit |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYWEEKNO |N/A |N/A |N/A |N/A |N/A |N/A |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYYEARDAY |Limit |Limit |Limit |N/A |N/A |N/A |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYMONTHDAY|Limit |Limit |Limit |Limit |N/A |Expand |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYDAY |Limit |Limit |Limit |Limit |Expand|Note 1 |Note 2|
+----------+--------+--------+-------+-------+------+-------+------+
|BYHOUR |Limit |Limit |Limit |Expand |Expand|Expand |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYMINUTE |Limit |Limit |Expand |Expand |Expand|Expand |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYSECOND |Limit |Expand |Expand |Expand |Expand|Expand |Expand|
+----------+--------+--------+-------+-------+------+-------+------+
|BYSETPOS |Limit |Limit |Limit |Limit |Limit |Limit |Limit |
+----------+--------+--------+-------+-------+------+-------+------+
Note 1: Limit if BYMONTHDAY is present; otherwise, special expand
for MONTHLY.
Note 2: Limit if BYYEARDAY or BYMONTHDAY is present; otherwise,
special expand for WEEKLY if BYWEEKNO present; otherwise,
special expand for MONTHLY if BYMONTH present; otherwise,
special expand for YEARLY.