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","$time"]},format:"%d/%m/%YT%H:%M"}}}},{$project:{date:1,parts:{$dateToParts:{date:"$date"}}}},{$match:{"parts.hour":{$gte:10}}}])