Aggregate by Day produces duplicate day results - node.js

Question

My mongodb aggregation query should return daily, weekly, monthly, yearly data within specific date ranges.
Example: let's assume dates between 1st Feb to 1st March.
the data for daily should be: 1st, 2nd, 3rd..
Weekly period will be: 1st Feb, 8th Feb, 15th Feb, 22nd Feb ..
monthly period will be 1st Feb, 1st march ..
Look at example below:
Let's say my API accepts: startDate, endDate, interval as body params.
req.body will be something like this:
{
startDate: "",
endDate: "",
platform: "",
interval: "daily" // could be "weekly", "monthly", "yearly"
}
These params will be passed to my model where I have some aggregation code which will be mentioned below:
MessagesSchema.statics.totalMessages = ( startDate, endDate, platform, interval ) => {
return Messages.aggregate([{
$match: {
platform: platform,
timestamp: {
$gte: new Date(startDate),
$lte: new Date(endDate)
}
}
},
{
$project: {
timestamp: {
$dateToString: {
format: '%Y-%m-%d',
date: '$timestamp'
}
}
}
},
{
$group: {
_id: {
timestamp: '$timestamp'
},
count: {
$sum: 1
}
}
},
{
$sort: {
'_id.timestamp': 1
}
}
]).exec();
Let's assume Weekly data from 1st Feb 2019 - 1st March 2019;
expected result:
[
{
"_id": {
"timestamp": "2019-02-01"
},
"count": 2
},
{
"_id": {
"timestamp": "2019-02-08"
},
"count": 2
},
{
"_id": {
"timestamp": "2019-02-15"
},
"count": 2
}
]
actual result:
[
{
"_id": {
"timestamp": "2019-02-01"
},
"count": 2
},
{
"_id": {
"timestamp": "2019-03-02"
},
"count": 2
},
{
"_id": {
"timestamp": "2019-03-02"
},
"count": 2
}
]