Convert date difference to years to calculate age in MongoDB

Update: MongoDB 5.0 solution, will consider leap years as well

db.collection.aggregate([  { $addFields:    { age: { $dateDiff: { startDate: "$dob", endDate: "$$NOW", unit: "year" } } }  }])

Update: We can just combine the aggregation operators. Note that this solution won't give accurate result as it does not consider leap years

db.getCollection('person').aggregate( [ {     $project: {         date:"$demographics.DOB",         age: {             $divide: [{$subtract: [ new Date(), "$Demographics.DOB" ] },                     (365 * 24*60*60*1000)]        }      } } ] )

Old solution with $let

I was able to solve the issue with $let expression

db.getCollection('person').aggregate( [ {     $project: {         item: 1,         date:"$demographics.DOB",         age: {             $let:{                vars:{                    diff: {                         $subtract: [ new Date(), "$demographics.DOB" ]                     }                },                in: {                    $divide: ["$$diff", (365 * 24*60*60*1000)]                }            }        }      } } ] )

The accepted answer is incorrect by as many days as there are a leap years that passed since the person was born. Here's a more correct way to calculate age:

{$subtract:[   {$subtract:[{$year:"$$NOW"},{$year:"$dateOfBirth"}]},   {$cond:[      {$gt:[0, {$subtract:[{$dayOfYear:"$$NOW"},      {$dayOfYear:"$dateOfBirth"}]}]},      1,      0   ]}]}

Starting in Mongo 5.0, it's a perfect use case for the new $dateDiff aggregation operator:

// { "dob" : ISODate("1990-03-27") }// { "dob" : ISODate("2008-08-07") }db.collection.aggregate([  { $addFields:    { age: { $dateDiff: { startDate: "$dob", endDate: "$$NOW", unit: "year" } } }  }])// { "dob" : ISODate("1990-03-27"), "age" : 31 }// { "dob" : ISODate("2008-08-07"), "age" : 13 }

This nicely fits the age scenario since (quoting the documentation) durations produced by $dateDiff are measured by counting the number of times a unit boundary is passed. For example, two dates that are 18 months apart would return 1 year difference instead of 1.5 years.

Also note that $$NOW is a variable that returns the current datetime value.