Database
[{_id:ObjectId("6188a5283543f7cc2f77c73f"),branch_id:1,borrowers_list:{0:{borrowed_dt:ISODate("2021-08-15T06:00:00.000+00:00"),card_no:"ID000067",return_dt:ISODate("2021-08-25T06:00:00.000+00:00")}}}]
Query
db.collection.aggregate([{$project:{Date_diff:{$dateDiff:{startDate:"$borrowers_list.0.borrowed_dt",endDate:"$borrowers_list.0.return_dt",unit:"day"}}}}])