How to aggregate by year-month-day on a different timezone

Question

I have a MongoDB whom store the date objects in UTC. Well, I want to perform aggregation by year,month day in a different timezone (CET).

doing this, works fine for

Answer 1

You can provide the timezone to the date operators starting in 3.6.

Replace the timezone with your timezone.

{
  "$group":{
    "_id":{
      "year":{"$year":{"date":"$tDate","timezone":"America/Chicago"}},
      "month":{"$month":{"date":"$tDate","timezone":"America/Chicago"}},
      "dayOfMonth":{"$dayOfMonth":{"date":"$tDate","timezone":"America/Chicago"}}
    },
    "count":{"$sum":1}
  }
}

Answer 2

After searching for hours, this is the solution that worked for me. It is also very simple. Just convert the timezone by subtracting the timezone offset in milliseconds.

25200000 = 7 hour offset // 420 min * 60 sec * 1000 mili

$group: {
    _id = { 
        year: { $year : [{ $subtract: [ "$timestamp", 25200000 ]}] }, 
        month: { $month : [{ $subtract: [ "$timestamp", 25200000 ]}] }, 
        day: { $dayOfMonth : [{ $subtract: [ "$timestamp", 25200000 ]}] }
    },
    count = { 
        $sum : 1
    }
};

Answer 3

The solution with timezone is a good one, but in version 3.6 you can also format the output using timezone, so, you get the result ready for use:

{
"$project":{
    "year_month_day": {"$dateToString": { "format": "%Y-%m-%d", "date": "$tDate", "timezone": "America/Chicago"}}
},
"$group":{
    "_id": "$year_month_day",
    "count":{"$sum":1}
}
}

Make sure that your "$match" also considers timezone, or else you will get wrong results.

Answer 4

Mongo stores the dates in UTC, so this is the procedure to get them in other zone

• check that mongo saves the dates in UTC, insert some records etc.
• get timezone offset with moment-timezone.js eg moment().tz('Europe/Zagreb').utcOffset() functions, for your specified timezone
• Prepare $gte and $lte for $match stage (eg user input for dates 1.1.2019 - 13.1.2019.):
  • If offset is positive subtract() those seconds in $match stage; If offset is negative add() those seconds in $match stage
• Then normalize the dates (because $match stage will return them in UTC) to your zone like this: -if timezone offset is positive add() those seconds in $project stage; -if timezone offset is negative subtract() those seconds in $project stage.
• $group goes last, this is important (because we want to group normalized results, and not $match-ed)

Basically it is this: shift input(s) to $match(UTC), and then normalize to your timezone.

Answer 5

I'm not an expert on CET and its relation to UTC, but the following code (for the shell) should do a proper conversion (adding an hour) to a MongoDB date type:

db.dates.aggregate(
  {$project: {"tDate":{$add: ["$tDate", 60*60*1000]}, "eventCount":1, "customer":1}}
)

If you run that project command before the rest of your pipeline, the results should be in CET.

Answer 6

MongoDB's documentation suggests that you save the timezone offset alongside the timestamp:

var now = new Date();
db.data.save( { date: now,
                offset: now.getTimezoneOffset() } );

This is of course not the ideal solution – but one that works, until we have in MongoDb's aggregation pipeline a proper $utcOffset function.