Thanks Chris,
It works like a charm.
http://server:8983/solr/collection/select?arrivalRange=121YEARS&json.facet={%22NEW%20ARRIVALS%22:{%22start%22:NOW/DAY-${arrivalRange},%20%22sort%22:%22index%22,%22type%22:%22range%22,%22field%22:%22pdp_activation_date_dt%22,%22gap%22:%22%2B${arrivalRange}%22
The JSON based query APIs (including JSON Faceting) use (and unfortunately
subtly different) '${NAME}' syntax for dereferencing variables in the
"body" of a JSON data structure...
https://lucene.apache.org/solr/guide/8_5/json-request-api.html#parameter-substitution-macro-expansion
...but note
