How to deal with the timezone issue when storing dates in utc using mongod?

Question

I have a mongodb collection where each document has some attributes and a utc timestamp. I need to pull out data from the collection and use the aggregation framework becaus

Answer 1

Every approach suggested above works perfectly fine, but since there is a new version of mongodb, from 2.6 you can use $let in the aggregation framework, this will let you create variables on the fly, thus avoiding the need to $project before grouping. Now you could create a variable with $let that will hold the localized time, and use it in the $group operator.

Something like:

db.test.aggregate([
   {$group: { 
        _id: { 
             $let: { 
                 vars: {  
                     local_time: { $subtract: ["$date", 10800000]} 
                 }, 
                 in: { 
                    $concat: [{$substr: [{$year: "$$local_time"}, 0, 4]}, 
                              "-", 
                              {$substr: [{$month: "$$local_time"}, 0, 2]}, 
                              "-", 
                              {$substr: [{$dayOfMonth: "$$local_time"}, 0, 2]}]
                 }
              }
         }, 
         count: {$sum: 1}
     }
 }])

Notice that you use $let inside definition of a block/variable, and the value of that block/variable is the returned value of the subexpression "in", where the above defined vars are used.