By Period Minus

Subtype of Date Spec

The expected date(s) must be defined indirectly through 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 subtracting the defined time interval(s) from certain context-dependent reference date(s).
The financial meaning of the context-dependent reference date(s) is described in the key(s) associated with the supplied time interval(s).