Reversing date conversion using specific format in MongoDB - mongodb

Question

I convert the date to a string so that I can use it for various grouping related work. However in certain groupings the reverse conversion fails.
db.test.aggregate({
$addFields: {
date: "$date",
grouped_format: {
"$dateToString": {
date: '$date',
format: "%Y-%m",
timezone: 'Europe/London',
}
}
}
}, {
$addFields: {
converted: {
"$dateFromString": {
dateString: '$grouped_format',
format: "%Y-%m", // if i comment this out, it works,
timezone: 'Europe/London',
}
}
}
})
The error I get is
{
"message" : "an incomplete date/time string has been found, with elements missing: '2016-01'",
"ok" : 0,
"code" : 241,
"codeName" : "ConversionFailure",
"name" : "MongoError"
}
If I comment the format under $dateFromString, I get the desired result.
{
"_id" : ObjectId("60ca299f10949130abbe5c8e"),
"date" : ISODate("2014-04-21T00:00:00.000Z"),
"grouped_format" : "2014-04",
"converted" : ISODate("2014-03-31T23:00:00.000Z") // London is one hour ahead due to DST so this is correct. -> start of the month
},
{
"_id" : ObjectId("60ca2aaf10949130abbe5c8f"),
"date" : ISODate("2021-01-02T00:00:00.000Z"),
"grouped_format" : "2021-01",
"converted" : ISODate("2021-01-01T00:00:00.000Z") // London is even with UTC so this is correct. -> start of the month
}
Unfortunately, I want to be able to specify any custom format in $dateToString and have it be converted back.
As an example, if the custom format is "%G-%V"which is the ISO week year and ISO week number, specifying the format in $dateToString and $dateFromString works correctly.
I've tried the following formats and here are the results. As you can see, the No. 4. in the table does not specify the minute but it still works fine. However any variation of the date from No.6. to 8. fails.
If I can't reliably reverse the value then I will need to create a special function that covers all edge cases but to do that I need to understand why this is happening in the first place.
No
Format
Works/Fails
1
'%Y-%m-%dT%H:%M:%S.%L'
Works
2
'%Y-%m-%dT%H:%M'
Works
3
'%Y-%m-%dT%H'
Works
4
'%Y-%m-%d'
Works
5
'%G-%V'
Works
6
'%Y-%m'
Fails
7
'%Y-%m-01'
Fails
8
'%Y-%m-00'
Fails
According to the documentation, the default formatter is "%Y-%m-%dT%H:%M:%S.%LZ" but specifying it explicitly also does not work and errors out. No string or null/undefined value seems to work while converting "%Y-%m" unless you omit the format string entirely in $dateFromString. This makes the behaviour very strange.