Used with derivatives when an event is express as a month-year with optional day or month or week of month.
YYYYMM (e.g. 199903)
YYYYMMDD (e.g. 20030323)
YYYYMMwN (e.g. 200303w2) for week
A specific date can be appended to the month-year. For instance, if multiple event types exist in the same Year and Month, but actually at a different time, a value can be appended, such as "w" or "w2" to indicate week. Likewise, the day of monty (0-31) can be appended to indicate a specific event date.