Grouping the time-line is a useful capability for aggregates in temporal databases (+R1,+R3).
Partitioning is inappropriate because the valid-time elements may overlap; they do not necessarily form a partition since they may not cover the time-line. One example of valid-time grouping is to divide the time-line into years, based on the Gregorian calendar. Then for each year, compute the count of the tuples which overlap that year.
There is no existing term for this concept. There is no grouping attribute in valid-time grouping, since the grouping does not depend on attribute values, but instead on valid times.
Valid-time grouping may occur before or after value partitioning.