First park-local day this field was reported.
Newest park-local day this field appears in the ARCHIVE. This is not the same as the last day the field was reported, and it does NOT mean the field has stopped: the archive is written a day or two behind live data, so a field being published right now still has a last two days in the past. Every active field looks the same as a withdrawn one here. Use this to know how far back the archive goes and how current it is, not to decide whether a field is still live — GET /v1/entity/{id}/live answers that directly.
One live-data field's recorded span for an entity, both ends inclusive.