Elasticsearch sorting buckets of dates with script
I believe you are close, but have to return some value instead of 1 for the "future" dates.
Try substituting 1 for 1.0/doc['start_date'].value so it is inversely proportional to date (larger date values are further in the future).
Full example as above:
sort: [ { _script: { type: 'number', script: { lang: 'painless', inline: "if (doc['item_date'].value > params.current_time) {return 1.0/doc['start_date'].value;} else {return 0}", params: { current_time, }, }, order: 'desc', }, }, { item_date: { order: 'desc' } },]