[{"_id":ObjectId("63b730d71833eb85b96539ce"),"city":"Frankfurt a.M.","department_id":101,"employees":[{"_id":ObjectId("63b730dd1833eb85b9653dbb"),"age":28,"awards":1}],"name":"HR","projects":[{"_id":ObjectId("63b730dd1833eb85b96539dc"),"budget":5250100.98,"department_id":101,"end_date":new Date("1995-04-01"),"start_date":new Date("1990-04-23"),"title":"nhcdruimcimbytgnkklid"},{"_id":ObjectId("63b730dd1833eb85b96539ee"),"budget":5062704.46,"department_id":101,"end_date":new Date("1991-03-01"),"start_date":new Date("1990-10-11"),"title":"jqdewbvkvpeggckpj"},{"_id":ObjectId("63b730dd1833eb85b96539f9"),"budget":1858309.73,"department_id":101,"end_date":new Date("1993-04-01"),"start_date":new Date("1991-02-07"),"title":"urwwnrxzcgikuo"}]}]

db.collection.aggregate([{$match:{department_id:101}},{$unwind:"$projects"},{$group:{_id:null,averageTime:{$avg:{$dateDiff:{startDate:"$projects.start_date",endDate:"$projects.end_date",unit:"hour"}}}}},{$project:{_id:0,numDays:{$trunc:["$averageTime",1]}}}])