For one of my collections, which must remain unix timestamp instead of isodate, I usually convert the timestamp with new Date(unix_timestamp).

Now I need the new Date(ts) in an aggregation. (the example is in PHP)

'$project' => array ( 'day' => '$new Date(ts)', ...

'$group' => array ( "_id" => array('day' => '$day)', ...),...

does not arrive in the results. The field "day" in the result is missing entirely.

How to get this converted within the aggregation?

mongodb aggregation-framework
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