How does one properly increment many dates in mongoDB?

The problem is that my $exists query is (obviously, on second look) in the wrong place. Documents were being returned that, surely enough, didn't include my_date.

Here's the patched up function, which works as expected.

var incrementDates = function() {
  db.blah.find({ ... , my_date : { $exists : true } ).forEach(function(doc) {
    db.blah.update(
       { _id     : doc._id }
     , { $set : { my_date : new Date(doc.my_date.getTime() + 86400000) }}
    );
  });
}

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

// { "date" : ISODate("2020-04-05T07:14:17.802Z"), "x" : "y" }
db.collection.updateMany(
  { date : { $exists : true } },
  [{ $set: { date: { $dateAdd: { startDate: "$date", unit: "day", amount: 1 } } } }]
)
// { "date" : ISODate("2020-04-06T07:14:17.802Z"), "x" : "y" }

• The first part { date : { $exists : true } } is the match query, filtering which documents to update (in our case all documents having the date field).

• The second part [{ $set: { date: { $dateAdd: { startDate: "$date", unit: "day", amount: 1 } } } }], updates the value of the date field by adding ($dateAdd) 1 (amount) day (unit) to $date (startDate).

Starting in Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on its own value; thus avoiding inefficient find/foreach patterns.

Also, you were looking at the $inc operator to add a day, but now that we can use an aggregation pipeline as an update, the $add operator can be used:

// { "date" : ISODate("2020-04-05T07:14:17.802Z"), "x" : "y" }
db.collection.updateMany(
  { date : { $exists : true } },
  [{ $set: { date: { $add: ["$date", 24*60*60000] } } }]
)
// { "date" : ISODate("2020-04-06T07:14:17.802Z"), "x" : "y" }

• The first part { date : { $exists : true } } is the match query, filtering which documents to update (in our case all documents having the date field).

• The second part [{ $set: { date: { $add: ["$date", 24*60*60000] } } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias of $addFields. Then any aggregation operator can be used within the $set stage; in our case a simple $addition between the existing date and the representation of a day in milliseconds.