Database
[{month:"January",start:ISODate("2021-01-01"),end:ISODate("2021-01-31")},{month:"February",start:ISODate("2021-02-01"),end:ISODate("2021-02-28")},{month:"March",start:ISODate("2021-03-01"),end:ISODate("2021-03-31")},]
Query
db.collection.aggregate([{$project:{days:{$dateDiff:{startDate:"$start",endDate:"$end",unit:"day"}},_id:0}}])