Mongoose question

test.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var assert = require('assert')

console.log('\n===========');
console.log('    mongoose version: %s', mongoose.version);
console.log('========\n\n');


mongoose.set('debug', true);
mongoose.connect('localhost', 'dates');
mongoose.connection.on('error', function () {
    console.error('connection error', arguments);
});


// user model
var userSchema = Schema({ name: 'string' , dob: Date});
var User = mongoose.model('User', userSchema);

mongoose.connection.on('open', function () {
    User.create([{ name: 'mark' , dob: new Date(1970,0,1)}, { name: 'ann' , dob: new Date(1972,0,1)}], function(err){
        if (err) return done(err);
        // Find
        var findCommand = '{"dob":"1970-01-01"}'
        User.find(JSON.parse(findCommand),function(err,result){
            if (err) return done(err);
            console.log("Find: "+JSON.stringify(result));
            // Aggregate
//            var aggCommand = '[{"$match":{"name":"mark"}},{"$project":{"name":1}}]'    // works fine
            var aggCommand = '[{"$match":{"dob":"1970-01-01"}},{"$project":{"name":1}}]'  // returns empty array
            User.aggregate(JSON.parse(aggCommand),function(err,result){
                if (err) return done(err);
                console.log("Aggregate: "+JSON.stringify(result));
                done();
            })
        })
    })
});

function done (err) {
    if (err) console.error(err.stack);
    mongoose.connection.db.dropDatabase(function () {
        mongoose.connection.close();
    });
}