By Period Plus

Subtype of Date Spec

The expected date(s) are implied by one or more time intervals, i.e. one or more objects of type Period or any of its direct subtypes
The date(s) is(are) then constructed dynamicaly when needed by adding the defined time interval(s) to certain context-dependent reference date(s).
The financial meaning of the context-dependent reference dates is described in the key(s) associated with the supplied time interval(s).