[{id:1,creationdate:"2012-11-05",completiondate:"2012-11-06"},{id:1,creationdate:"2012-11-05",completiondate:"2012-11-06"},{id:2,creationdate:"2012-11-05",completiondate:"2012-11-07"}]

db.collection.aggregate([{$project:{creationdate:{$dateFromString:{dateString:"$creationdate",format:"%Y-%m-%d"}},completiondate:{$dateFromString:{dateString:"$completiondate",format:"%Y-%m-%d"}}}},{$project:{difference:{$subtract:["$completiondate","$creationdate"]}}},{$group:{_id:null,average:{$avg:"$difference"}}},{$project:{_id:0,dayAverage:{$divide:["$average",86400000]}}}])