How to Group MongoDB Records by Hour in Code (Node.js)?

Working with Data
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1

Hi all,

I’m working on a Node.js app that logs user actions with timestamps in MongoDB. I want to generate hourly activity summaries — basically group records by hour of the day (e.g. 13:00–14:00, 14:00–15:00, etc.) and count how many actions happened in each hour.

Here’s a simplified structure of my documents:

{
  "userId": "abc123",
  "action": "login",
  "timestamp": ISODate("2025-07-13T13:45:00Z")
}

In my aggregation pipeline, I’m using $hour: “$timestamp” but the groupings feel off, possibly due to timezone differences. I’m also not sure if I should handle the hour extraction in the aggregation pipeline or in the application code.

Is there a preferred way to group by local hour using MongoDB aggregation within Node.js (e.g., with the native driver or Mongoose)?

Any suggestions or sample code would be appreciated!

Thanks,
Jhonn Mick

2

Hi @Jhonn_Marie, so as you’ve said, the $hour operator is off due to timezone differences, so to get around this, you can use the $dateTrunc operator and it works from MongoDB 5.0+.

This is how it would work in your use-case -

db.collection.aggregate([
  {
    $group: {
      _id: {
        $dateTrunc: {
          date: "$timestamp",
          unit: "hour",
          timezone: "Your/Timezone"
        }
      },
      count: { $sum: 1 }
    }
  },
  {
    $sort: { _id: 1 } // sort by hour
  }
])

This link also gives you more information to how $dateTrunc works -

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3

Thanks a ton, Michael! That’s exactly what I needed. I didn’t realize $dateTrunc handled timezone offsets so cleanly, super helpful. I’ll give this a try with my local timezone and see how it goes. Really appreciate you including the example too!

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