Database
[{"Item":"Test1","millisecond":188760000},{"Item":"Test2","millisecond":23280000},{"Item":"Test3","millisecond":128820000},]
Query
db.collection.aggregate([{$project:{"Item":1,"hours":{$floor:{$divide:["$millisecond",3600000]}},"minutes":{$floor:{$divide:[{$mod:["$millisecond",3600000]},60000]}}}},{$project:{"Item":1,"millisecond":{$concat:[{$toString:"$hours"},":",{$toString:"$minutes"},":00"]}}}])