Database
[{"_id":1,"StartDate":ISODate("2021-03-20T15:31:34.846Z")},{"_id":2,"StartDate":new Date()}]
Query
db.collection.aggregate([{$match:{$expr:{$and:[{$gte:["$StartDate",{"$toDate":"2021-03-01T00:00:00.0000000Z"}]},{$lt:["$StartDate",{"$toDate":"2021-04-01T00:00:00.0000000Z"}]}]}}}])