Database
[{"geojson":5,"entity_status_update":ISODate("2021-12-28T10:16:58.000+00:00")},{"geojson":7,"entity_status_update":ISODate("2021-12-03T10:16:58.000+00:00")},{"geojson":2,"entity_status_update":ISODate("2021-11-08T10:16:58.000+00:00")}]
Query
db.collection.aggregate([{"$addFields":{"days_to_add":"$geojson"}},{"$project":{"days_to_add":1,"deadline":{"$dateAdd":{"startDate":"$entity_status_update","unit":"day","amount":"$days_to_add"}}}}])