- Just a start date;
- Just an end date (from ERN onwards 4.2 this will be impossible);
- A start and an end date;
- Just a start date-time;
- Just an end date-time (from ERN onwards 4.2 this will be impossible); and
- A start and an end date-time.(1)
(1) Note, it is technically also possible to have a period tag with no start and no end. This approach is, however, not acceptable; from ERN 4.2 onwards this will be impossible.