[{"uID":"12345678","midNightTimeStamp":NumberInt(1645381800),"waterDispensed":NumberInt(53)},{"uID":"12345678","midNightTimeStamp":NumberInt(1645641000),"waterDispensed":NumberInt(30)}]

db.collection.aggregate([{$group:{_id:null,ar:{$push:"$$ROOT"},mind:{"$min":"$midNightTimeStamp"},maxd:{"$max":"$midNightTimeStamp"}}},{$project:{ar:{$map:{input:{$range:["$mind",{"$sum":["$maxd",86400]},86400]},as:"dateInRange",in:{$let:{vars:{dateIndex:{"$indexOfArray":["$ar.midNightTimeStamp","$$dateInRange"]}},in:{$cond:{if:{$ne:["$$dateIndex",-1]},then:{$arrayElemAt:["$ar","$$dateIndex"]},else:{midNightTimeStamp:"$$dateInRange","waterDispensed":NumberInt(0)}}}}}}}}},{$unwind:"$ar"},{$project:{_id:0,"waterDispensed":"$ar.waterDispensed",midNightTimeStamp:"$ar.midNightTimeStamp","Date":{$toDate:{"$multiply":["$ar.midNightTimeStamp",1000]}}}}])