How to get the difference of two ISO String dates with an aggregation MongoDB query? - mongodb

Question

I have a collection called biosignals in my MongoDB, in which there exist entries that represent physical activity (e.g. walking).
Each such entry has a 'start_date_time' and an 'end_date_time', which are ISO strings (e.g. 2017-04-26T07:12:09.463Z).
I want to do the following query, where I group the physical activity entries by day and I calculate the total duration of activity for each day.
db.biosignals.aggregate([
{
$match: {
"name": "physical-activity"
}
},
{
$project: {
duration: {
"$subtract": [new Date("$end_date_time"), new Date("$start_date_time")]
},
date: {
$substr: [ "$start_date_time", 0, 10]
}
}
},
{
$group: {
_id: "$date",
total: { $sum: "$duration" }
}
},
{
$sort: {
_id: 1
}
}
])
However, I only get 0 as a result, as shown here:
{ "_id" : "2017-04-24", "total" : NumberLong(0) }
{ "_id" : "2017-04-25", "total" : NumberLong(0) }
{ "_id" : "2017-04-26", "total" : NumberLong(0) }
{ "_id" : "2017-04-27", "total" : NumberLong(0) }
If, instead, I hardcode the dates (e.g. start_date_time = 2017-04-26T07:12:08.463Z and end_date_time = 2017-04-26T07:12:09.463Z, that is one second difference), I get the expected result:
{ "_id" : "2017-04-24", "total" : NumberLong(16000) }
{ "_id" : "2017-04-25", "total" : NumberLong(3000) }
{ "_id" : "2017-04-26", "total" : NumberLong(7000) }
{ "_id" : "2017-04-27", "total" : NumberLong(12000) }
How could I fix that?
Thank you very much!