Database
[{"admin":{"emailSentDate":"11-07-2023"}},{"admin":{"emailSentDate":"11-07-2022"}}]
Query
db.collection.find({$expr:{$gte:[{$dateFromString:{dateString:"$admin.emailSentDate",format:"%d-%m-%Y"}},{$dateSubtract:{startDate:"$$NOW",unit:"month",amount:6,}}]}})