[{_id:1,item:"apple",qty:5,order_date:new Date("2018-03-10")},{_id:2,item:"pie",qty:10,order_date:new Date("2018-03-12")},{_id:3,item:"ice cream",qty:2,price:"4.99",order_date:"2018-03-05"},{_id:4,item:"almonds",qty:5,price:5,order_date:"2018-03-05 +10:00"}]

db.collection.aggregate([{$addFields:{convertedDate:{$toDate:"$order_date"}}}])