Database
[{"yyyymmdd":20220220}]
Query
db.collection.aggregate([{"$addFields":{"ymd":{"$convert":{"input":{"$concat":[{"$substr":[{"$toLong":"$yyyymmdd"},0,4]},"-",{"$substr":[{"$toLong":"$yyyymmdd"},4,2]},"-",{"$substr":[{"$toLong":"$yyyymmdd"},6,2]}]},"to":"string"}}}},{"$addFields":{"week":{"$week":{"$toDate":"$ymd"}}}}])