The current default behavior of MongoDB's $dateToString aggregation expression is incorrect because it violates ISO 8601. (It tacks on a "Z" to the end of the string by default, which implies that it is in UTC [1], but in actually it's in the timezone of the local server.)

This ticket proposes a number of routes to change the default behavior of $dateToString to bring it into compliance with ISO 8601.

[1] https://en.wikipedia.org/wiki/ISO_8601#Coordinated_Universal_Time_(UTC)

Description of Linked Ticket

When using the $dateToString aggregation operator on an ISODate value, specifying a timezone but not a format string, an invalid/inaccurate ISO8601 date string is returned.

Specifically: 2023-04-26T12:06:17.194Z when using something like:

db.collection.aggregate([{ "$addFields": {"txDateString": { "$dateToString": {date: "$transactionDateTS", timezone: "America/New_York"} } }} ])`

The root issue is that the "Z" categorically implies a timezone of UTC (+00:00)... But, in this case, that's absolutely incorrect and, technically, makes it an invalid ISO8601 date string. (I suppose one could argue that it's syntatically valid - but it's certainly misleading and could result in major issues if another program were to parse the date string as it would interpret the time as "Zulu"/UTC because of the "Z".) The inclusion of the "Z" when explicitly specifying a timezone is certainly unexpected...

At a minimum, when a format string is unspecified and a date/time in UTC is converted to another time zone, the "Z" should be left off. Ideally, the UTC offset (obtained from the timezone conversion) should be included as per ISO8601 – i.e. in this case, 2023-04-26T12:06:17.194-04:00 (at the time of this writing, America/New_York is on Eastern Daylight Time (EDT) which is UTC-04:00.)

Related issues appear to include: SERVER-33173 and SERVER-28610