Database
[{"date":"20/03/2019","time":"15:00"},{"date":"20/03/2018","time":"10:00"},{"date":"20/03/2017","time":"01:00"}]
Query
db.collection.aggregate([{$project:{_id:0,date:{$dateFromString:{dateString:{$concat:["$date","T","$time"]},format:"%d/%m/%YT%H:%M"}}}},{$match:{date:{$gte:ISODate("2018-03-20")}}}])