Find avg difference in dates stored as strings - mongodb

Question

I have a Mongo database and I have stored dates as strings. Per document I have a field called "creationdate" and a field called "completiondate". The dates format is "YYYY-MM-dd" (ex "2011-12-18"). Even I can execute simple aggregation like greaterThan, greaterThanEqual, I cannot find the difference in dates, which I have to find to calculate the average days difference between completion and creation date.
The above query I have to write it on spring-boot with MongoTemplate if it is possible.
I am trying something like this but it doesn't work.
Aggregation aggregation = Aggregation.newAggregation(
Aggregation.match(Criteria.where("creationdate").gte(date1).lte(date2).andOperator(Criteria.where("completiondate").ne(""))),
Aggregation.project("servicerequesttype").and(DateOperators.DateFromString.fromStringOf("completiondate").withFormat("%Y-%m-%d")).minus(DateOperators.DateFromString.fromStringOf("creationdate").withFormat("%Y-%m-%d")).as("diff"),
Aggregation.group("servicerequesttype").avg("diff").as("average")
);
date1, date2 are given strings like "2011-01-01"

Answer 1

Is this what you are looking for?
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
]
}
}
}
])
I have created interactive demo here: https://mongoplayground.net/p/wGRw12m3UbB
Hope it helps :)

Answer 2

Spring-Boot
Aggregation aggregation = Aggregation.newAggregation(
Aggregation.match(Criteria.where("creationdate").gte(date1).lte(date2).andOperator(Criteria.where("completiondate").ne(""))),
Aggregation.project("servicerequesttype").and(DateOperators.DateFromString.fromStringOf("creationdate").withFormat("%Y-%m-%d")).as("creationdate").and(DateOperators.DateFromString.fromStringOf("completiondate").withFormat("%Y-%m-%d")).as("completiondate"),
Aggregation.project("servicerequesttype").and("completiondate").minus("creationdate").as("difference"),
Aggregation.group("servicerequesttype").first("servicerequesttype").as("servicerequesttype").avg("difference").as("temp"),
Aggregation.project("servicerequesttype").and("temp").divide(86400000).as("average")
);