Implement a $dateToString aggregation operator.

e.g. : $dateToString:

{format: "<format string>", date: "<$field>"}

Modifiers should be mostly similar to strftime, but with the addition of milliseconds.

All units should be zero-padded to the maximum number of digits for that unit.

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I'd like to propose an aggregation operator that supports formatting date objects as strings (using the strftime function).

This comes very handy when working with time-series data.

For example: if you want to aggregate documents by a specific hour, you'd have to do this: {$group: {_id: {year: {$year: '$date'}, month: {$month: '$date'}, day:

{'$dayOfMonth': '$date'}

, hour: {$hour: '$date'}}}}

Here's how you can do it with dateFormat: {$group: {_id: {$dateFormat: ['$date', '%Y-%m-%d %H']}}}

In addition to the shorter syntax, the results look much nicer (-:

I've already implemented the feature and the tests (I'll add a link after the ticket is created).

This is my first contribution to the MongoDB server code. I'll open a pull request referencing this issue. Please tell me if I'm doing anything wrong.