The newly added DateRangePrefixTree (DRPT) encodes terms in a fashion amenable to faceting by meaningful time buckets. The motivation for this feature is to efficiently populate a calendar bar chart or heat-map. It's not hard if you have date instances like many do but it's challenging for date ranges.
Internally this is going to iterate over the terms using seek/next with TermsEnum as appropriate. It should be quite efficient; it won't need any special caches. I should be able to re-use SPT traversal code in AbstractVisitingPrefixTreeFilter. If this goes especially well; the underlying implementation will be re-usable for geospatial heat-map faceting.